coot-model-building

name	coot-model-building
description	Best Practices for Model-Building Tools and Refinement

Key Lessons from Chain A Refinement Session

Workflow Checklist - Follow This for Model-Building and Refinement

Best practices for fixing any issue:

1. ☐ ALWAYS recentre first — every time you move to a new residue or problem, call this BEFORE anything else: coot.set_go_to_atom_chain_residue_atom_name(chain, resno, "CA") or coot.set_rotation_centre(x, y, z) for a map position. The user is watching the screen. If you switch targets without recentring, they will be looking at the wrong place while you work elsewhere. This is confusing and unacceptable.
2. ☐ Check current metrics (Rama/correlation/overlaps)
3. ☐ Make checkpoint if trying something experimental
4. ☐ Apply fix
5. ☐ Re-check metrics to confirm improvement
6. ☐ If worse, restore checkpoint

Jiggle Fitting a Whole Chain

When to use jiggle fitting

Use fit_chain_to_map_by_random_jiggle_and_blur() when a whole chain needs to be placed or re-placed in the map — before any residue-level refinement. Note, however, that the fit is only local. If the whole chain is (say) 10 or more Angstroms away from the real position, then fit_chain_to_map_by_random_jiggle_and_blur() will not work. The per-residue density correlation graph is the key diagnostic: if the mean correlation across the whole chain is near zero with high variance (many residues below 0.4, many negative), the chain is in the wrong place and jiggle fitting is the right first step.

Do not attempt residue-level fixes (rotamers, pepflips, refinement) until the chain is correctly placed. Those tools cannot recover a globally misplaced chain.

Diagnosing misplacement from the correlation graph

Before jiggling, always get a fresh per-residue correlation to use as a true baseline:

stats = coot.map_to_model_correlation_stats_per_residue_range_py(
    imol, chain_id, imol_map, 1, 0)
corrs = [entry[1][1] for entry in stats[0]]
mean_corr = sum(corrs) / len(corrs)
n_below = sum(1 for c in corrs if c < 0.4)
print(f"Mean correlation: {mean_corr:.3f}, residues below 0.4: {n_below}/{len(corrs)}")

Interpretation:

• Mean > 0.7, few residues below 0.4 → chain is well-placed, proceed to residue-level work
• Mean 0.4–0.7, moderate number below 0.4 → chain placed but needs refinement
• Mean near zero, high variance, many negative → chain is in the wrong place, jiggle fit first

Why negative correlations occur in cryo-EM maps: cryo-EM maps are Fourier-based (no F000 term), so the mean density is zero. Negative density exists between atomic features as a mathematical consequence of the reconstruction — not a physical reality. A misplaced chain within the molecular envelope samples this real structured signal (both positive peaks and negative troughs) from the wrong part of the map, producing scattered correlations around zero. This is distinct from sitting in solvent, which gives low-magnitude, low-variance correlations.

Running the jiggle fit

Set the refinement map first, then use the blur variant for cryo-EM:

coot.set_imol_refinement_map(imol_map)

result = coot.fit_chain_to_map_by_random_jiggle_and_blur(
    imol,          # model molecule
    chain_id,      # e.g. "N"
    n_trials,      # 200-500; more trials = better chance of finding global optimum
    jiggle_scale,  # 1.0-2.0; scale of random perturbations
    blur_factor    # 100-200; Fourier filtering makes the map smoother, helping
                   # the fit escape local minima and find the correct placement
)
print(f"Jiggle fit score: {result:.3f}")

Return values: the function returns a score (higher = better fit) or a large negative number (e.g. −100, −999) when it cannot improve on the current placement. These large negative returns are not errors — they mean the current position is already a local optimum, or the function had difficulty scoring. Do not treat them as failures without checking the actual correlation afterwards.

Blur factor guidance: the blur smooths the map by a something like a Fourier filter, effectively low-pass filtering it. A higher blur factor produces a smoother map with broader features, which helps the chain find the correct region before fine-grained fitting. For cryo-EM maps, blur factors of 100–200 are typical starting points.

Confirming the result

Always re-run the per-residue correlation after jiggling and compare to the baseline:

stats_after = coot.map_to_model_correlation_stats_per_residue_range_py(
    imol, chain_id, imol_map, 1, 0)
corrs_after = [entry[1][1] for entry in stats_after[0]]
mean_after = sum(corrs_after) / len(corrs_after)
n_below_after = sum(1 for c in corrs_after if c < 0.4)
print(f"AFTER  — mean: {mean_after:.3f}, below 0.4: {n_below_after}")

A successful jiggle fit for a correctly placed chain should bring the mean correlation above 0.6, with most residues above 0.7. Remaining poor-fitting residues in loop regions are expected and addressed by subsequent residue-level refinement.

Visualising the before/after result

Use the coot-inline-graphs skill to render a before/after bar chart with secondary structure overlays. Fetch secondary structure from PDBe (/api/pdb/entry/secondary_structure/{pdb_id}) rather than relying solely on Coot's header secondary structure detection, which may miss strands — particularly single- residue assignments and short strands in nanobody/VHH-type folds.

Complete workflow summary

# 1. Get fresh baseline correlation (after any manual moves, before jiggle)
stats_before = coot.map_to_model_correlation_stats_per_residue_range_py(
    imol, chain_id, imol_map, 1, 0)
corrs_before = [e[1][1] for e in stats_before[0]]
print(f"BEFORE — mean: {sum(corrs_before)/len(corrs_before):.3f}")

# 2. Set refinement map and jiggle fit
coot.set_imol_refinement_map(imol_map)
result = coot.fit_chain_to_map_by_random_jiggle_and_blur(imol, chain_id, 500, 1.0, 200.0)
print(f"Jiggle score: {result:.3f}")  # large negative = ignore, check correlation anyway

# 3. Get after correlation and compare
stats_after = coot.map_to_model_correlation_stats_per_residue_range_py(
    imol, chain_id, imol_map, 1, 0)
corrs_after = [e[1][1] for e in stats_after[0]]
print(f"AFTER  — mean: {sum(corrs_after)/len(corrs_after):.3f}")

# 4. Only proceed to residue-level work once mean > 0.6

Refinement Best Practices

1. Make Checkpoints Before Model Changes

CRITICAL: Always create a checkpoint before making significant model changes.

Use make_backup_checkpoint() before any operation that might need to be reverted:

• Adding/deleting residues
• Adding ligands or waters
• Major refinement operations
• Any experimental model building

# Create a named checkpoint before risky operation
checkpoint_idx = coot.make_backup_checkpoint(0, "before adding OXT")

# Try the operation
coot.add_OXT_to_residue(0, "A", 93, "")
result = coot.refine_residues_py(0, [["A", 93, ""]])

# Check if it worked - if not, restore
if result_is_bad:
    coot.restore_to_backup_checkpoint(0, checkpoint_idx)

Why checkpoints are better than undo:

• apply_undo() only steps back one operation at a time
• Checkpoints let you jump back to a specific point
• Named checkpoints are self-documenting
• Multiple checkpoints allow comparing different approaches

# Compare two different approaches
checkpoint_before = coot.make_backup_checkpoint(0, "original state")

# Try approach 1
coot.auto_fit_best_rotamer(0, "A", 42, "", "", 1, 1, 0.01)
results = coot.refine_residues_py(0, [["A", 41, ""], ["A", 42, ""], ["A", 43, ""]])
score_approach1 = check_correlation(0, "A", 42)
checkpoint_approach1 = coot.make_backup_checkpoint(0, "after approach 1")

# Restore and try approach 2
coot.restore_to_backup_checkpoint(0, checkpoint_before)
coot.pepflip(0, "A", 42, "", "")
results = coot.refine_residues_py(0, [["A", 41, ""], ["A", 42, ""], ["A", 43, ""]])
score_approach2 = check_correlation(0, "A", 42)

# Keep the better result
if score_approach1 > score_approach2:
    coot.restore_to_backup_checkpoint(0, checkpoint_approach1)

MANDATORY: Complete Validation Workflow

CRITICAL: You MUST check ALL validation metrics before AND after EVERY fix.

Fixing only one problem (e.g., Ramachandran) while leaving others (rotamer, density fit) is a FAILED fix. A residue is only "fixed" when ALL metrics are acceptable.

Before fixing ANY residue:

1. ALWAYS center on it: coot.set_go_to_atom_chain_residue_atom_name(chain, resno, "CA")
2. ALWAYS check ALL of these:
   • Ramachandran probability (from all_molecule_ramachandran_score_py)
   • Rotamer score (from rotamer_graphs_py)
   • Density correlation - both all-atom and side-chain (from map_to_model_correlation_stats_per_residue_range_py)
   • Atom overlaps involving this residue (from molecule_atom_overlaps_py)

After fixing ANY residue:

3. ALWAYS re-check ALL the same metrics, Sometime residues/issues are just not fixable (that's what makes refinement and model-building tricky).
4. Try to make the following true (but knowing that it might not actually be possible)

• Ramachandran probability > 0.02 (preferably > 0.1)
• Rotamer score > 1.0% (preferably > 5%)
• Density correlation > 0.7 (all-atom and side-chain, preferably > 0.8)
• No severe clashes (< 2.0 Å cubed overlap volume)

If ANY metric is still bad after your first fix:

5. MUST try additional fixes:
   • Bad rotamer → auto_fit_best_rotamer(), and try experiment with following that up with refine_residues_py() for that residue and its upstream and downstream neighbours (if any).
   • Poor density fit → try alternative rotamers, check for missing atoms
   • Persistent clashes → refine with the addition of spatial neighbours using residues_near_residue_py()
6. NEVER declare a residue "fixed" based on only one metric improving
7. ALWAYS re-validate after each additional fix

Acceptable Reasons to Stop (without perfect metrics):

• You've tried multiple approaches and documented them
• The best achievable metrics are still recorded
• You've created checkpoints to compare approaches
• You explain why the residue remains problematic (e.g., poor density, crystal contact)

Example of CORRECT Workflow

# 1. ALWAYS center on problem residue first
coot.set_go_to_atom_chain_residue_atom_name("A", 41, "CA")

# 2. Get ALL metrics BEFORE
rama_data = [r for r in coot.all_molecule_ramachandran_score_py(0)[5:]
             if r[1] == ['A', 41, '']][0]
rama_prob_before = rama_data[2]

rotamer_data = [r for r in coot.rotamer_graphs_py(0)
                if r[0] == 'A' and r[1] == 41][0]
rotamer_score_before = rotamer_data[3]

corr_data = [s for s in coot.map_to_model_correlation_stats_per_residue_range_py(0, "A", 1, 1, 0)[0]
             if s[0][1] == 41][0]
correlation_before = corr_data[1][1]

overlaps_before = [o for o in coot.molecule_atom_overlaps_py(0, 30)
                   if (o['atom-1-spec'][1:3] == ['A', 41] or
                       o['atom-2-spec'][1:3] == ['A', 41])]

print(f"BEFORE: Rama={rama_prob_before:.4f}, Rotamer={rotamer_score_before:.2f}%, Corr={correlation_before:.3f}, Clashes={len(overlaps_before)}")

# 3. Apply first fix (e.g., pepflip for backbone)
coot.pepflip(0, "A", 41, "", "")
coot.refine_residues_py(0, [["A", 40, ""], ["A", 41, ""], ["A", 42, ""], ["A", 43, ""]])

# 4. Check ALL metrics AFTER first fix
rama_prob_after = [r for r in coot.all_molecule_ramachandran_score_py(0)[5:] 
                   if r[1] == ['A', 41, '']][0][2]
rotamer_score_after = [r for r in coot.rotamer_graphs_py(0) 
                       if r[0] == 'A' and r[1] == 41][0][3]
correlation_after = [s for s in coot.map_to_model_correlation_stats_per_residue_range_py(0, "A", 1, 1, 0)[0] 
                     if s[0][1] == 41][0][1][1]

print(f"AFTER:  Rama={rama_prob_after:.4f}, Rotamer={rotamer_score_after:.2f}%, Corr={correlation_after:.3f}")

# 5. If rotamer or correlation still bad, DON'T STOP - fix them!
if rotamer_score_after < 1.0:
    print("Rotamer still bad - trying auto_fit_best_rotamer")
    coot.auto_fit_best_rotamer(0, "A", 41, "", "", 1, 1, 0.01)
    coot.refine_residues_py(0, [["A", 40, ""], ["A", 41, ""], ["A", 42, ""]])

    # 6. ALWAYS re-check after additional fixes
    rotamer_score_final = [r for r in coot.rotamer_graphs_py(0) 
                           if r[0] == 'A' and r[1] == 41][0][3]
    correlation_final = [s for s in coot.map_to_model_correlation_stats_per_residue_range_py(0, "A", 1, 1, 0)[0] 
                         if s[0][1] == 41][0][1][1]
    print(f"FINAL:  Rotamer={rotamer_score_final:.2f}%, Corr={correlation_final:.3f}")

# 7. Only NOW can you move to the next residue

Example of WRONG Workflow (DO NOT DO THIS)

# ❌ WRONG: Checking only Ramachandran
coot.pepflip(0, "A", 41, "", "")
coot.refine_residues_py(0, [["A", 40, ""], ["A", 41, ""], ["A", 42, ""]])
rama_after = coot.all_molecule_ramachandran_score_py(0)[5][39][2]
print(f"Ramachandran improved to {rama_after}")
# MOVES ON without checking rotamer or density fit - WRONG!

# ❌ WRONG: Not centering on residue
# Goes straight to fix without set_go_to_atom_chain_residue_atom_name()

# ❌ WRONG: Not checking metrics before the fix
# How do you know if it improved if you don't know what it was before?

# ❌ WRONG: Declaring success with bad rotamer
rama = 0.30  # Good!
rotamer = 0.0001  # TERRIBLE!
correlation = 0.59  # POOR!
print("Residue fixed!")  # NO IT ISN'T!

Why This Matters

A residue with:

• ✅ Good Ramachandran (0.30)
• ❌ Terrible rotamer (0.01%)
• ❌ Poor density fit (0.59)

is NOT fixed. The side chain is clearly wrong. The backbone geometry might be OK, but the model is still incorrect.

ALL metrics must be acceptable before moving on.

2. Extend Selection Around Problem Residues

Don't refine problem residues in isolation - include neighboring residues for context.

• ❌ Bad: refine_residues_py(0, [["A", 41, ""]]) - Often fails to correct the model
• ✅ Good: refine_residues_py(0, [["A", 40, ""], ["A", 41, ""], ["A", 42, ""], ["A", 43, ""]])

Recommended approach:

• For single problem residue: include ±1 or ±2 neighbors
• For consecutive problem residues: include ±1 neighbor on each end
• Larger regions (±3-4 residues) can sometimes help severe issues

Example from session:

• Residues 41-42 had severe Ramachandran outliers
• Refining just 41-42 failed
• Refining 40-43 succeeded: Residue 41 improved from p=0.00004 to p=0.308

Neighboring Residues:

• You can use coot.residues_near_residue_py() to find residues that are close in space but distant in sequence, so that they can be added to the residue selection for refinement.

3. Include Spatial Neighbours, Not Just Sequence Neighbours

Critical insight: Residues that are close in 3D space affect each other during refinement, even if they're far apart in sequence.

Coot's refinement includes spatially neighbouring atoms in the non-bonded contact interactions, but only the selected residues can move during minimization. If a nearby (but unselected) residue is in the wrong position, it will "push" your selected residues away via non-bonded contact penalties - potentially pushing them out of correct density to avoid the clash with the incorrectly-placed neighbour.

Diagnostic workflow:

1. Check for clashes after refinement:

overlaps = coot.molecule_atom_overlaps_py(0, 50)
for o in overlaps:
    spec1, spec2 = o['atom-1-spec'], o['atom-2-spec']
    vol = o['overlap-volume']
    if vol > 0.5:  # Significant clash
        print(f"{spec1[1]}/{spec1[2]} {spec1[4]} - {spec2[1]}/{spec2[2]} {spec2[4]}: {vol:.2f}")

2. If a problem residue clashes with a distant residue, fix the distant residue first:

# Example: A/2 has poor correlation (0.13) and clashes with A/89
# First fix A/89:
coot.auto_fit_best_rotamer(0, "A", 89, "", "", 1, 1, 0.01)
results = coot.refine_residues_py(0, [["A", 88, ""], ["A", 89, ""], ["A", 90, ""]])

# Then re-refine A/2 INCLUDING A/89 as a spatial neighbour:
results = coot.refine_residues_py(0, [["A", 1, ""], ["A", 2, ""], ["A", 3, ""], ["A", 89, ""]])
# A/2 correlation improved: 0.13 → 0.81

Why this matters:

• Coot's refinement "feels" spatial neighbours via non-bonded contact terms
• But only selected residues can move during minimization
• A/89 was in a wrong position (correlation 0.050) and pushing A/2 away
• A/2 moved out of its correct density to reduce the non-bonded penalty with A/89
• Fixing A/89 first put it in the right place, so it no longer pushed A/2 incorrectly

Real example:

Before: A/2 correlation = 0.131, A/89 correlation = 0.050
        A/2 CA ↔ A/89 CZ clash: 1.06 Ų

After fixing A/89 first, then refining together:
        A/2 correlation = 0.805, A/89 correlation = 0.928
        No clash

4. Iterative Refinement Strategy

Sometimes multiple rounds of refinement with different selections help:

1. First pass: Refine larger region to establish general geometry
2. Second pass: Refine smaller region to fine-tune specific problem
3. Check validation after each step
4. Restore checkpoint if results get worse

Example workflow:

# Create checkpoint first!
checkpoint = coot.make_backup_checkpoint(0, "before iterative refinement")

# First: larger region
results_1 = coot.refine_residues_py(0, [["A", i, ""] for i in range(40, 44)])
check_validation()  # Did it help?

# Second: targeted refinement
results_2 = coot.refine_residues_py(0, [["A", 41, ""], ["A", 42, ""], ["A", 43, ""]])
check_validation()  # Better or worse?

# If worse:
coot.restore_to_backup_checkpoint(0, checkpoint)

5. Measure Before and After

Always validate changes objectively using:

• Ramachandran probabilities
• Density correlation (all-atom and side-chain)
• Geometry statistics

def check_residue_validation(imol, chain_id, resno):
    """Check both Ramachandran and density correlation"""
    # Get Ramachandran
    rama_data = coot.all_molecule_ramachandran_score_py(imol)
    residue_data = rama_data[5]
    rama_score = None
    for r in residue_data:
        if r[1][0] == chain_id and r[1][1] == resno:
            rama_score = r[2]
            break

    # Get density correlation
    corr_data = coot.map_to_model_correlation_stats_per_residue_range_py(
        imol, chain_id, 1, 1, 1
    )
    all_atom_corr = None
    sidechain_corr = None

    for r in corr_data[0]:
        if r[0][1] == resno:
            all_atom_corr = r[1][1]
            break

    for r in corr_data[1]:
        if r[0][1] == resno:
            sidechain_corr = r[1][1]
            break

    return {
        'residue': resno,
        'rama_prob': rama_score,
        'all_atom_corr': all_atom_corr,
        'sidechain_corr': sidechain_corr
    }

# Usage with checkpoint
checkpoint = coot.make_backup_checkpoint(0, "before refinement test")
before = check_residue_validation(0, "A", 41)

coot.refine_residues_py(0, [["A", 40, ""], ["A", 41, ""], ["A", 42, ""], ["A", 43, ""]])
coot.accept_moving_atoms_py()
after = check_residue_validation(0, "A", 41)

# Compare and decide
if after['all_atom_corr'] > before['all_atom_corr']:
    # Keep it!
    pass
else:
    # Revert to checkpoint
    coot.restore_to_backup_checkpoint(0, checkpoint)

6. Use Checkpoints and Undo Liberally

Don't be afraid to revert changes:

• make_backup_checkpoint() / restore_to_backup_checkpoint() - for jumping back to a specific state
• apply_undo() - for stepping back one operation at a time

Use checkpoints when:

• Starting a new model-building task
• About to try something experimental
• Before a series of related operations

Use undo when:

• The last single operation made things worse
• Quick single-step revert needed

7. Auto-fit Rotamer for Side-chain Issues

For poor side-chain density correlation, try auto_fit_best_rotamer() first:

# Create checkpoint first
checkpoint = coot.make_backup_checkpoint(0, "before rotamer fitting")

# Check if it's a side-chain problem
validation = check_residue_validation(0, "A", 89)
if validation['sidechain_corr'] < 0.5:
    # Try auto-fit rotamer
    score = coot.auto_fit_best_rotamer(0, "A", 89, "", "", 1, 1, 0.01)
    
    if score > 0:  # Positive score is good
        # Check improvement
        after = check_residue_validation(0, "A", 89)
        if after['sidechain_corr'] > validation['sidechain_corr']:
            # Success! (e.g., 0.034 → 0.900)
            pass
        else:
            coot.restore_to_backup_checkpoint(0, checkpoint)
    else:
        # Negative score means failure
        coot.restore_to_backup_checkpoint(0, checkpoint)

8. Set Refinement to Synchronous Mode

Always call this at the start to make refinement complete immediately:

coot.set_refinement_immediate_replacement(1)

Without this, refinement may be asynchronous and difficult to control programmatically.

9. Navigate to Residue Before Working

Bring residue to screen center so you can watch the refinement:

coot.set_go_to_atom_molecule(0)
coot.set_go_to_atom_chain_residue_atom_name("A", 41, "CA")

This helps with:

• Visual inspection of the problem
• Seeing the refinement in real-time
• Verifying the result makes geometric sense

10. Flipping peptides

If the Ramachandran Plot is poor, try using coot.pepflip(imol, chain_id, res_no, ins_code, alt_conf) followed by a refinement of the residues in the extended region.

11. Flipping side-chains terminal Chi-angle

If the Rotamer score is poor, try using coot.do_180_degree_side_chain_flip() to improve the Rotamer score. It is occasionally useful.

12. Delete Waters Displaced by Newly Built Residues

CRITICAL: After adding any new residues, always check for and delete water molecules that are now too close to the new protein atoms.

Waters are placed by earlier refinement cycles that had no knowledge of atoms you are about to build. Once new residues are placed, any water within ~3.5 Å is almost certainly spurious — it was filling density that now belongs to the protein model.

Always do this immediately after building new residues, before refinement.

# After building new residues (e.g. A21-A25), check each for clashing waters
newly_built = [21, 22, 23, 24, 25]
waters_to_delete = set()

for resno in newly_built:
    neighbours = coot.residues_near_residue_py(0, ["A", resno, ""], 3.5)
    for n in neighbours:
        chain, nresno, inscode = n[0], n[1], n[2]
        name = coot.residue_name_py(0, chain, nresno, inscode)
        if name == "HOH":
            waters_to_delete.add((chain, nresno, inscode))
            print(f"Water {chain}{nresno} within 3.5A of new A{resno} - will delete")

for chain, resno, inscode in waters_to_delete:
    coot.delete_residue(0, chain, resno, inscode)
    print(f"Deleted HOH {chain}{resno}")

Why 3.5 Å? This is a generous hydrogen-bond distance. Any water oxygen closer than this to a protein heavy atom is physically incompatible with the new model.

Don't just check for hard clashes — even waters that don't show up in molecule_atom_overlaps_py() should be deleted if they are within 3.5 Å, because the overlap check uses van der Waals radii and may miss close-but-not-overlapping waters that are nonetheless chemically unreasonable.

13. Building N-terminal Extensions with Enforced Secondary Structure

When building backwards (toward the N-terminus) into a known secondary structure element, use add_terminal_residue_using_phi_psi() with the target φ/ψ angles and enforce the conformation further by refining with set_secondary_structure_restraints_type().

α-helix: φ = −57°, ψ = −47° β-strand: φ = −120°, ψ = +120°

Complete workflow for N-terminal helix extension:

# Step 1: Checkpoint
cp = coot.make_backup_checkpoint(0, "before N-terminal extension")

# Step 2: Build residues backwards one at a time with helix phi/psi
phi, psi = -57.0, -47.0
current_nterm = 26  # existing N-terminal residue of fragment
for i in range(4):  # build 4 residues: 25, 24, 23, 22
    r = coot.add_terminal_residue_using_phi_psi(0, "A", current_nterm, "auto", phi, psi)
    current_nterm -= 1

# Step 3: IMMEDIATELY check for and delete displaced waters
newly_built = list(range(current_nterm + 1, 26))
waters_to_delete = set()
for resno in newly_built:
    for n in coot.residues_near_residue_py(0, ["A", resno, ""], 3.5):
        if coot.residue_name_py(0, n[0], n[1], n[2]) == "HOH":
            waters_to_delete.add((n[0], n[1], n[2]))
for chain, resno, inscode in waters_to_delete:
    coot.delete_residue(0, chain, resno, inscode)
    print(f"Deleted displaced water {chain}{resno}")

# Step 4: Mutate from ALA placeholders to correct sequence
coot.mutate_residue_range(0, "A", 22, 25, "NLLND")  # sequence from UniProt etc.

# Step 5: Autofit rotamers for mutated residues
for resno in newly_built:
    coot.auto_fit_best_rotamer(0, "A", resno, "", "", 1, 1, 0.01)

# Step 6: Refine with helix restraints - always turn off afterwards
coot.set_secondary_structure_restraints_type(1)
coot.refine_residues_py(0, [["A", r, ""] for r in range(22, 28)])
coot.accept_moving_atoms_py()
coot.set_secondary_structure_restraints_type(0)  # ALWAYS turn off when done

Key rules for secondary structure restraints:

• Always call set_secondary_structure_restraints_type(0) after refinement to clear them
• Extend the refinement selection 1-2 residues beyond the newly built region into the existing chain, to provide proper geometric context at the junction
• Helix restraints (type 1), strand restraints (type 2), no restraints (type 0)

coot.set_secondary_structure_restraints_type(1)  # alpha helix
coot.set_secondary_structure_restraints_type(2)  # beta strand
coot.set_secondary_structure_restraints_type(0)  # off - ALWAYS restore to this

14. Rebuilding a Residue with a Missing Main-chain N Atom

If a residue is missing a main-chain atom — most commonly the amide N — the ordinary atom/side-chain filling tools silently do nothing. They rebuild atoms onto an existing N–CA–C frame, so a residue with no N gives them nothing to build from. Do not infer "unfixable" from these no-ops; they are the wrong tools for a missing main-chain atom.

Observed no-ops on a CYS that had only CA, CB, C, O (no N, no SG):

• fill_partial_residues(imol) — whole-molecule fill; skips the residue
• fill_partial_residue(imol, chain, resno, ins) — single-residue fill; skips it
• mutate_and_autofit_residue_range(...) — cannot seat a side chain without the N frame
• spin_N_py(imol, spec, angle) — repositions an existing N (it interchanges the N and CB positions); it does not create one, so it is a no-op when no N exists. Its docstring ("on N-terminal addition the N ends up pointing the wrong way") is the signpost: the N is created by terminal-residue building, and spin_N_py is only the follow-up that orients it.

The fix: delete the broken residue and rebuild it with add_terminal_residue(), which regenerates a complete main-chain (N included) by fitting to the map. It comes back as an ALA stub; then restore the correct residue type and side chain.

imol, imap = 0, 1
coot.set_refinement_immediate_replacement(1)
cp = coot.make_backup_checkpoint(imol, "before A72 mainchain-N rebuild")

# 1. Delete the residue that is missing its main-chain N
coot.delete_residue(imol, "A", 72, "")

# 2. Rebuild from the neighbour. Building forward from 71 (or backward from
#    73) regenerates a full main-chain, CREATING the N atom. add_terminal_residue
#    returns 1 on success, 0 on failure. The residue returns as an ALA stub
#    (N, CA, C, O, CB).
ok = coot.add_terminal_residue(imol, "A", 71, "auto", 1)

# 3. Confirm the N now exists before proceeding
atoms = [a[0][0].strip() for a in coot.residue_info_py(imol, "A", 72, "")]
assert "N" in atoms, "add_terminal_residue did not create the N atom"

# 4. Restore the real residue type and seat its side chain
coot.mutate_and_autofit_residue_range(imol, "A", 72, 72, "C")   # "C" = CYS

# 5. Only if the newly built N points the wrong way, spin it into place
#    (spin_N works now that an N exists; 120 deg swaps N and CB)
# coot.spin_N_py(imol, ["A", 72, ""], 120.0)

# 6. Refine with neighbours to seat the new backbone into density
coot.refine_residues_py(imol, [["A", 71, ""], ["A", 72, ""], ["A", 73, ""]])

Verify main-chain geometry (not just the validation metrics) — a correctly rebuilt N gives an ideal peptide bond and Cα–N distance:

import math

def atom_dist(imol, c1, r1, a1, c2, r2, a2):
    def xyz(c, r, a):
        for at in coot.residue_info_py(imol, c, r, ""):
            if at[0][0].strip() == a:
                return at[2]
        return None
    p, q = xyz(c1, r1, a1), xyz(c2, r2, a2)
    return math.dist(p, q) if p and q else None

# targets: C(i-1)-N(i) ~1.33 A ; N-CA ~1.46 A
print(atom_dist(0, "A", 71, "C", "A", 72, "N"))
print(atom_dist(0, "A", 72, "N", "A", 72, "CA"))

Notes:

• A rebuilt complete residue often shows a lower density correlation than the broken fragment did, because the fragment's correlation was computed over fewer atoms. Compare like with like: a complete residue at 0.62 in a mobile loop is a better model than a 4-atom stub scoring 0.68.
• This is the general recipe for any missing main-chain atom, not just N: delete and rebuild rather than trying to patch a single atom into an incomplete frame.

NCS-Related Chains: Always Use Copy, Never Rebuild

CRITICAL RULE: When the user asks you to fix a region in one chain using a corrected region in another chain, ALWAYS use copy_residue_range_from_ncs_master_to_others() first. Never attempt to rebuild the target chain manually.

Trigger Phrases That Must Invoke This Approach

Any of these mean: use the NCS copy tool immediately:

• "Use the fixed B chain to fix chain A"
• "Apply the same fix to the other chain"
• "The equivalent region in chain X needs fixing"
• "Propagate this fix to the NCS-related chain"
• "Now do the same for chain A/B/C..."
• Any mention of NCS, related chains, or symmetry-equivalent regions after a fix

NCS Copy Workflow

# 1. Check NCS is detected — do this first
ncs_chains = coot.ncs_chain_ids_py(imol)
print(ncs_chains)  # e.g. [['A', 'B']] — master chain listed first

# 2. Set the fixed chain as NCS master
coot.ncs_control_change_ncs_master_to_chain_id(imol, "B")  # B is the fixed chain

# 3. Copy the fixed region to all NCS-related chains using the NCS operator
coot.copy_residue_range_from_ncs_master_to_others(imol, "B", start_resno, end_resno)

# 4. Refine the target chain in its own density to let it settle
specs = [["A", r, ""] for r in range(start_resno - 1, end_resno + 2)
         if coot.residue_name_py(imol, "A", r, "") not in [None, "False", ""]]
coot.refine_residues_py(imol, specs)

Why this is always better than manual rebuilding:

• The NCS operator correctly handles the rotation and translation between chains — NCS-related chains are in different parts of the unit cell, so you cannot simply copy coordinates without applying the NCS transformation
• It propagates correct backbone geometry that is already known to be good
• It is fast — one function call instead of dozens of pepflips and refinement cycles
• Refinement against the target chain's own density then handles the small differences between NCS-related chains

Only fall back to manual rebuilding if:

• ncs_chain_ids_py() returns False (no NCS detected)
• The chains have genuinely different sequences in the region of interest
• The NCS copy produces clearly wrong geometry (check correlations immediately after)

---

Per-Atom Density Probing: The Most Powerful Diagnostic

Use density_at_point() per backbone atom to diagnose backbone problems before attempting any fix. This is more informative than per-residue correlation alone.

def probe_backbone(imol, imol_map, imol_diff, chain, resnos):
    sigma      = coot.map_sigma_py(imol_map)
    sigma_diff = coot.map_sigma_py(imol_diff)
    backbone   = [" N  ", " CA ", " C  ", " O  "]

    for resno in resnos:
        atoms = coot.residue_info_py(imol, chain, resno, "")
        if not isinstance(atoms, list): continue
        name = coot.residue_name_py(imol, chain, resno, "")
        coords = {a[0][0]: a[2] for a in atoms}
        for atname in backbone:
            xyz = coords.get(atname)
            if xyz is None: continue
            x, y, z = xyz
            sig_main = coot.density_at_point(imol_map,  x, y, z) / sigma
            sig_diff = coot.density_at_point(imol_diff, x, y, z) / sigma_diff
            flag = " << LOW" if sig_main < 0.8 else ""
            if sig_main < 0.0: flag = " << NEGATIVE"
            print(f"  {chain}/{resno} {name} {atname.strip():4s}  "
                  f"{sig_main:.2f}σ  diff={sig_diff:+.2f}σ{flag}")

Interpreting Per-Atom Density Values

Pattern	Diagnosis	Fix
O near 0σ, N/CA/C good	Peptide bond flipped — carbonyl O in empty space	pepflip() at this residue
CA near 0σ or negative	Atom genuinely in wrong position	Register shift or delete/rebuild
Large negative diff at CA	Atom displaced — strong negative difference density	Delete and rebuild or NCS copy
Large positive diff nearby	Missing atoms or unmodelled density	Build missing residues
All atoms < 1σ	Entire residue misplaced	Delete and rebuild

The pepflip signature: near-zero or negative carbonyl O with good N, CA, C. This is the most common backbone error and the fastest to fix.

# If O < 0.5σ but CA > 2σ — it's a pepflip
coot.pepflip(imol, chain_id, resno, "", "")
specs = [[chain_id, r, ""] for r in range(resno-2, resno+3)
         if coot.residue_name_py(imol, chain_id, r, "") not in [None, "False", ""]]
coot.refine_residues_py(imol, specs)
# Re-probe to confirm O has moved into density

---

Mainchain vs Sidechain Correlation: Distinguish Before Acting

Always check mainchain and sidechain correlation separately before deciding on a fix.

# atom_mask_mode: 0=all, 1=mainchain only, 2=sidechain only
residue_specs = [[chain_id, resno, ""]]

mc = coot.map_to_model_correlation_py(imol, residue_specs, [], 1, imol_map)
sc = coot.map_to_model_correlation_py(imol, residue_specs, [], 2, imol_map)
aa = coot.map_to_model_correlation_py(imol, residue_specs, [], 0, imol_map)

print(f"All-atom: {aa:.3f}  Mainchain: {mc:.3f}  Sidechain: {sc:.3f}")

Result	Conclusion	Fix
MC poor, SC good	Backbone error (pepflip, register)	Per-atom density probe, then pepflip
SC poor, MC good	Sidechain rotamer wrong	auto_fit_best_rotamer()
Both poor	Whole residue misplaced	Delete/rebuild or NCS copy

---

Register Shift Diagnosis

When a stretch of residues has consistently poor correlations flanked by good residues, always test for a register shift before attempting pepflips.

A sequence of consecutive Ramachandran outliers with poor density for the sidechains, but acceptable or even good coorelations for the mainchain atoms, is the hallmark of a register error, not individual residue problems.

How to Test for a Register Shift

# Test if B[n] matches A[n-1] (chain B one residue ahead of A)
for n in range(start, end):
    nb = coot.residue_name_py(imol, "B", n, "")
    na = coot.residue_name_py(imol, "A", n-1, "")
    match = "==" if nb == na else "!="
    print(f"B/{n} {nb}  vs  A/{n-1} {na}  {match}")

# Also compare phi/psi angles — a run of 3+ matching phi/psi confirms the shift
rama = coot.all_molecule_ramachandran_score_py(imol)
per_res = [r for r in rama[5] if r != -1]
rama_b = {r[1][1]: (r[0][0], r[0][1]) for r in per_res if r[1][0] == "B"}
rama_a = {r[1][1]: (r[0][0], r[0][1]) for r in per_res if r[1][0] == "A"}

for n in range(start, end):
    phi_b, psi_b = rama_b.get(n, (0, 0))
    phi_a, psi_a = rama_a.get(n-1, (0, 0))
    dphi = min(abs(phi_b-phi_a), 360-abs(phi_b-phi_a))
    dpsi = min(abs(psi_b-psi_a), 360-abs(psi_b-psi_a))
    match = "GOOD" if dphi < 30 and dpsi < 30 else "bad"
    print(f"B/{n} vs A/{n-1}: dphi={dphi:.0f} dpsi={dpsi:.0f}  {match}")

Also Verify Chain Geometry

import math
def dist(a, b): return math.sqrt(sum((a[i]-b[i])**2 for i in range(3)))

gap = dist(c_prev, n_next)
# ~1.33 A = direct peptide bond (no gap)
# ~3.8  A = one residue missing
# ~9.6  A = two residues missing

---

Spurious Inserted Residues: Delete and Renumber

A residue with near-zero or negative correlation, missing sidechain atoms, and a zero Ramachandran score, and highly distorted bonds and angles is almost certainly a phantom insertion placed to compensate for a genuine deletion elsewhere in the loop. This is called an "out of register" error - at some point in the model there will be a residue squeezed in to where it should not be and elsewhere there will be a residue that is stretched across where 2 residues should be.

Signs of a spurious residue:

• Correlation < 0.2 or negative
• Ramachandran score = 0.00000
• Missing sidechain atoms
• Flanking residues fit well
• CA→CA distance to neighbour is too short (~3.2 Å instead of ~3.8 Å)

The correct fix is delete + renumber, NOT pepflip + refine:

coot.make_backup_checkpoint(imol, "before delete spurious residue")
coot.delete_residue(imol, chain_id, resno, "")
coot.renumber_residue_range(imol, chain_id, resno+1, last_resno, -1)

# Fix residue types if sequence labels are now wrong
for n, target_restype in corrections.items():
    coot.mutate(imol, chain_id, n, "", target_restype)

for n in affected_resnos:
    coot.auto_fit_best_rotamer(imol, chain_id, n, "", "", imol_map, 1, 0.01)

specs = [[chain_id, r, ""] for r in range(resno-2, last_affected+2)
         if coot.residue_name_py(imol, chain_id, r, "") not in [None, "False", ""]]
coot.refine_residues_py(imol, specs)

After renumbering, always check:

1. Bond geometry: C→N ~1.33 Å, CA→CA ~3.8 Å
2. Sequence alignment vs the other NCS chain
3. Correlations throughout the corrected region

---

Anisotropic B-Factors: Always Handle Safely

B-factor fields can be lists (anisotropic) not floats (isotropic). Always use this helper — failing to do so causes a TypeError:

def get_b(atom):
    b = atom[1][1]
    return b[0] if isinstance(b, list) else b

---

Chain Break Geometry: Measure Before Building

import math
def dist(a, b): return math.sqrt(sum((a[i]-b[i])**2 for i in range(3)))

c_prev  = get_coords(imol, chain, resno_before)['C']
n_next  = get_coords(imol, chain, resno_after)['N']
cn_dist = dist(c_prev, n_next)
print(f"C->N: {cn_dist:.2f} A")
# ~1.33 A: direct bond — no residue missing, do not attempt to build
# ~3.8  A: one residue missing
# ~9.6  A: two residues missing

If C→N ≈ 1.33 Å, there is no room — the deletion is genuine, the model is correct.

---

Know When to Stop Automated Methods

When pepflips and rotamer fitting cycle without convergence after 2-3 attempts, stop. Use the per-atom density probe to diagnose the true problem.

Oscillation (O good → CA bad → O bad → CA good) means the residue is genuinely misplaced and automated tools cannot find the correct basin. Options:

• Use protein_db_loops_py() for proper fragment-database loop fitting
• Use NCS copy if a related chain is already correct (always prefer this)

Do not use multi_residue_torsion_fit_py as a substitute for proper loop fitting. It is useful for initial placement of manually built placeholder atoms only.

---

Loop Fitting with protein_db_loops_py

Use only when NCS copy is not available and the loop is genuinely missing (not a register error).

# Anchors must exist; missing residues need placeholder atoms first
specs = [
    [chain_id, anchor_start,   ""],  # last good residue before gap
    [chain_id, anchor_start+1, ""],  # last good residue before gap
    [chain_id, missing_1,      ""],  # built with placeholder atoms
    [chain_id, missing_2,      ""],  # built with placeholder atoms
    [chain_id, anchor_end,     ""],  # first good residue after gap
]

result = coot.protein_db_loops_py(
    imol, specs, imol_map,
    20,    # nfrags — 20 is thorough
    True   # preserve_residue_names
)

---

MANDATORY: Complete Validation Workflow

Check ALL metrics before AND after EVERY fix.

Before fixing ANY residue:

1. Center on it
2. Check Ramachandran, rotamer, all-atom corr, sidechain corr, overlaps
3. For backbone problems: check per-atom density with density_at_point()
4. For NCS structures: check if the related chain has the correct conformation

After fixing ANY residue:

• Ramachandran probability > 0.02 (preferably > 0.1)
• Rotamer score > 1.0% (preferably > 5%)
• Density correlation > 0.7 all-atom and sidechain (preferably > 0.8)
• No severe clashes (overlap volume < 2.0 ų)
• Backbone atoms all > 1σ in 2mFo-DFc map

Acceptable Reasons to Stop Without Perfect Metrics:

• Per-atom probe shows atoms are in correct density despite low per-residue correlation
• Remaining issues are at gap boundaries (Ramachandran nan is expected there)
• Residue is genuinely disordered (high B-factors consistent with low correlation)
• Multiple approaches documented and best achievable state recorded

---

Refinement Best Practices

Make Checkpoints Before Changes

checkpoint_idx = coot.make_backup_checkpoint(imol, "before pepflip A/262")
# ... make changes ...
# If worse: coot.restore_to_backup_checkpoint(imol, checkpoint_idx)
# If checkpoint fails: coot.set_undo_molecule(imol); coot.apply_undo()

Extend Selection Around Problem Residues

specs = [[chain_id, r, ""] for r in range(resno-2, resno+3)
         if coot.residue_name_py(imol, chain_id, r, "") not in [None, "False", ""]]
coot.refine_residues_py(imol, specs)

Sphere Refinement for Spatial Context

central = [chain_id, resno, ""]
neighbours = coot.residues_near_residue_py(imol, central, 4.5)
coot.refine_residues_py(imol, neighbours + [central])

Set Refinement to Synchronous Mode

coot.set_refinement_immediate_replacement(1)  # once at session start
coot.set_imol_refinement_map(imol_map)

Delete Waters Displaced by Newly Built Residues

for resno in newly_built_resnos:
    for n in coot.residues_near_residue_py(imol, [chain, resno, ""], 3.5):
        if coot.residue_name_py(imol, n[0], n[1], n[2]) == "HOH":
            coot.delete_residue(imol, n[0], n[1], n[2])

Secondary Structure Restraints

coot.set_secondary_structure_restraints_type(1)  # helix (phi=-57, psi=-47)
coot.set_secondary_structure_restraints_type(2)  # strand (phi=-120, psi=+120)
coot.refine_residues_py(imol, specs)
coot.set_secondary_structure_restraints_type(0)  # ALWAYS turn off afterwards

---

Mainchain Geometry Diagnostic: Bond, Angle and Omega Z-scores

Use this tool to identify distorted backbone geometry. It is fast, independent of the refinement environment, and often reveals register errors and misplaced residues that correlation and Ramachandran scores alone miss.

The omega torsion angle (the C-CA-C-N dihedral across the peptide bond, ideal = 180° for trans) is the most sensitive indicator. A wildly wrong omega (e.g. 9°, 60°, 123°) is definitive proof that the backbone is misplaced — it cannot be a real conformation.

Complete Diagnostic Function

import math

def dist(a, b):
    return math.sqrt(sum((a[i]-b[i])**2 for i in range(3)))

def angle_deg(a, b, c):
    ba = tuple(a[i]-b[i] for i in range(3))
    bc = tuple(c[i]-b[i] for i in range(3))
    cos_a = sum(ba[i]*bc[i] for i in range(3)) / (
        math.sqrt(sum(x*x for x in ba)) * math.sqrt(sum(x*x for x in bc)))
    return math.degrees(math.acos(max(-1.0, min(1.0, cos_a))))

def omega_deg(ca1, c1, n2, ca2):
    def cross(a, b):
        return (a[1]*b[2]-a[2]*b[1], a[2]*b[0]-a[0]*b[2], a[0]*b[1]-a[1]*b[0])
    def sub(a, b): return tuple(a[i]-b[i] for i in range(3))
    def norm(v):
        l = math.sqrt(sum(x*x for x in v))
        return tuple(x/l for x in v)
    def dot(a, b): return sum(a[i]*b[i] for i in range(3))
    b1 = sub(c1, ca1); b2 = sub(n2, c1); b3 = sub(ca2, n2)
    n1 = norm(cross(b1, b2)); n2v = norm(cross(b2, b3))
    m1 = cross(n1, norm(b2))
    return math.degrees(math.atan2(dot(m1, n2v), dot(n1, n2v)))

# Engh & Huber ideal values (sigma = expected standard deviation)
IDEAL = {
    'N-CA':   (1.458, 0.019),
    'CA-C':   (1.525, 0.021),
    'C-N':    (1.329, 0.014),   # peptide bond
    'C-O':    (1.231, 0.020),
    'N-CA-C': (111.2, 2.8),
    'CA-C-N': (116.2, 2.0),
    'C-N-CA': (121.7, 1.8),
    'CA-C-O': (120.8, 1.7),
}

def get_coords(imol, chain, resno):
    atoms = coot.residue_info_py(imol, chain, resno, "")
    if not isinstance(atoms, list) or atoms is False:
        return {}
    return {a[0][0].strip(): a[2] for a in atoms}

def check_mainchain_geometry(imol, chain, res_start, res_end, z_threshold=3.0):
    """
    Check mainchain bond lengths, angles and omega torsions against Engh & Huber ideals.
    Returns list of (residue_tag, parameter, ideal, actual, z_score, kind) tuples,
    sorted by |Z| descending.
    """
    distortions = []
    prev = None

    for resno in range(res_start, res_end + 1):
        name = coot.residue_name_py(imol, chain, resno, "")
        if not name or name in ["False", "", False]:
            prev = None
            continue
        c = get_coords(imol, chain, resno)
        if not c or "CA" not in c:
            prev = None
            continue

        tag = f"{chain}/{resno} {name}"

        # Intra-residue bonds
        for bond, (a1, a2) in [("N-CA",("N","CA")), ("CA-C",("CA","C")), ("C-O",("C","O"))]:
            if a1 in c and a2 in c:
                d = dist(c[a1], c[a2])
                ideal, sigma = IDEAL[bond]
                z = (d - ideal) / sigma
                if abs(z) >= z_threshold:
                    distortions.append((tag, bond, ideal, d, z, "bond"))

        # Intra-residue angles
        for ang_name, (a1,a2,a3) in [("N-CA-C",("N","CA","C")), ("CA-C-O",("CA","C","O"))]:
            if all(a in c for a in (a1,a2,a3)):
                ang = angle_deg(c[a1], c[a2], c[a3])
                ideal, sigma = IDEAL[ang_name]
                z = (ang - ideal) / sigma
                if abs(z) >= z_threshold:
                    distortions.append((tag, ang_name, ideal, ang, z, "angle"))

        # Inter-residue geometry (peptide bond to previous residue)
        if prev:
            prev_name = coot.residue_name_py(imol, chain, resno-1, "")
            ptag = f"{chain}/{resno-1} {prev_name}→{tag}"

            if "C" in prev and "N" in c:
                # Peptide C-N bond length
                d = dist(prev["C"], c["N"])
                ideal, sigma = IDEAL["C-N"]
                z = (d - ideal) / sigma
                if abs(z) >= z_threshold:
                    distortions.append((ptag, "C-N(pept)", ideal, d, z, "bond"))

                # Angles across the peptide bond
                if "CA" in prev:
                    ang = angle_deg(prev["CA"], prev["C"], c["N"])
                    ideal, sigma = IDEAL["CA-C-N"]
                    z = (ang - ideal) / sigma
                    if abs(z) >= z_threshold:
                        distortions.append((ptag, "CA-C-N", ideal, ang, z, "angle"))

                if "CA" in c:
                    ang = angle_deg(prev["C"], c["N"], c["CA"])
                    ideal, sigma = IDEAL["C-N-CA"]
                    z = (ang - ideal) / sigma
                    if abs(z) >= z_threshold:
                        distortions.append((ptag, "C-N-CA", ideal, ang, z, "angle"))

                    # Omega torsion — flag deviation from 180° > 15°
                    if "CA" in prev:
                        om = omega_deg(prev["CA"], prev["C"], c["N"], c["CA"])
                        dev = abs(abs(om) - 180.0)
                        if dev > 15.0:
                            # Express as Z using sigma~5° for trans peptides
                            distortions.append((ptag, "omega", 180.0, om, dev/5.0, "torsion"))

        prev = c

    distortions.sort(key=lambda x: -abs(x[4]))
    return distortions

Usage

# Check a region — results sorted worst first
dist_b = check_mainchain_geometry(imol, "B", 256, 268, z_threshold=3.0)

for res, param, ideal, actual, z, kind in dist_b:
    unit = "Å" if kind == "bond" else "°"
    flag = " ***" if abs(z) > 5 else ""
    print(f"  {res:35s}  {param:12s}  ideal={ideal:.3f}{unit}  actual={actual:.3f}{unit}  z={z:+.2f}{flag}")

Interpreting the Results

Distortion	Likely cause	Fix
omega far from 180° (> 20°)	Misplaced backbone / register error	Delete + renumber or NCS copy
omega near 0° (cis peptide)	May be genuine (rare) — check density	Visual inspection; pepflip if in empty density
N-CA-C angle severely wrong	Atom in completely wrong position	Per-atom density probe, then delete/rebuild
N-CA or CA-C bond short	Compressed geometry from register error	Delete + renumber
CA-C-N or C-N-CA angle wrong	Distorted peptide junction	Pepflip or rebuild

Omega outliers are the fastest route to finding register errors. A run of 3+ consecutive omega outliers in the same region is essentially diagnostic of a misplaced loop.

Always render the results as an interactive SVG widget

After running check_mainchain_geometry(), render the results using visualize:show_widget so each distorted residue is a clickable block that navigates to it or triggers a fix. Use c-red for |Z| > 5 or omega deviation > 20°, c-amber for |Z| 3–5. See the coot-essential-api skill for the widget rendering guidelines.

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Key Takeaways

For NCS structures: always check for a related chain that is correctly built before attempting manual rebuilding. One NCS copy call beats dozens of pepflips.

Use per-atom density probing before pepflipping. The pattern of which backbone atoms are in low density tells you exactly what is wrong and what fix to apply.

Distinguish mainchain from sidechain problems using atom_mask_mode=1 and 2 before deciding on a fix strategy.

Measure C→N and CA→CA distances before building. If C→N ≈ 1.33 Å there is no gap and no room to build — the deletion is genuine.

Context matters in refinement. Include neighbouring residues, both sequence and spatial neighbours, for best results.

Know when to stop. If automated methods cycle without convergence after 2-3 attempts, use the density probe to decide whether to delete, rebuild, or accept.

Key Takeaway

Context matters in refinement. Including neighboring residues provides the geometric and density context needed for refinement algorithms to find better solutions, especially for severe outliers.

Always checkpoint before changes. Use make_backup_checkpoint() before any significant model modification so you can easily revert if needed.

Delete displaced waters immediately after building new residues — don't wait until refinement reveals clashes.

Always turn off secondary structure restraints (coot.set_secondary_structure_restraints_type(0)) after the refinement that uses them, so they don't inadvertently affect subsequent work.