| name | bio-free-energy-calculations |
| description | Performs alchemical free-energy calculations including relative binding free energy (RBFE / FEP+) and absolute binding free energy (ABFE) via OpenFE, FEP+, GROMACS, AMBER pmemd, and OpenMM with explicit lambda window scheduling, soft-core potentials, REST2 enhanced sampling, MBAR/BAR analysis, and cycle closure validation. Compares ML alternatives (Boltz-2 affinity, DeepDock). Use when ranking analogs by binding affinity beyond docking accuracy, performing prospective lead optimization, or validating SAR predictions. |
| tool_type | mixed |
| primary_tool | OpenFE |
Version Compatibility
Reference examples tested with: OpenFE 1.7+, OpenMM 8.1+, GROMACS 2024+, AMBER pmemd 22+, alchemlyb 2.1+, pymbar 4.0+, RDKit 2024.09+.
Before using code patterns, verify installed versions match. If versions differ:
- Python:
pip show <package> then help(module.function) to check signatures
- CLI:
openfe --version; gmx --version; pmemd.cuda --version
If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.
Free Energy Calculations
Predict binding affinity differences (RBFE) or absolute binding affinities (ABFE) using alchemical free-energy methods. FEP+ (Schrödinger) is the commercial industry standard; OpenFE (Open Free Energy) is the open-source reference. Modern best practice achieves 1-2 kcal/mol RMSE vs experimental for well-set-up RBFE on rigid receptors. Boltz-2 affinity module (Wohlwend 2025) approaches FEP accuracy at 1000x speed on benchmarks, but FEP remains gold standard for production lead optimization.
For docking input poses, see chemoinformatics/virtual-screening. For pose validation before FEP, see chemoinformatics/pose-validation. For ML alternatives, see chemoinformatics/ml-docking-rescoring.
FEP Method Taxonomy
| Method | Cost / pair | Accuracy | Use case | Fails when |
|---|
| FEP+ (Schrödinger) | hours-days GPU | 1-2 kcal/mol RMSE | Commercial lead opt | License cost |
| OpenFE RBFE | hours-days GPU | comparable to FEP+ | Open-source RBFE | Setup automation less mature |
| OpenFE ABFE | days GPU | 2-3 kcal/mol RMSE | Absolute affinity | Slower; more setup care |
| GROMACS RBFE | hours-days GPU | 1-2 kcal/mol | Power users, custom setup | Manual setup is error-prone |
| AMBER pmemd RBFE | hours-days GPU | 1-2 kcal/mol | Tradition; force-field maturity | Manual setup |
| FEP-SPell-ABFE | days GPU | 2-3 kcal/mol | Automated ABFE | Limited adoption |
| QligFEP v2.1 | minutes-hours | 1.5-3 kcal/mol | Q-based ligand FEP | Less standard |
| MM/PBSA | minutes | 3-5 kcal/mol RMSE | Endpoint, fast | Limited accuracy; entropy missing |
| MM/GBSA | minutes | 3-5 kcal/mol RMSE | Endpoint, faster than PBSA | Same caveats |
| Boltz-2 affinity | seconds GPU | 0.66 Pearson on FEP subset | ML alternative; 1000x faster | Novel chemotypes |
| ALEPB / EE-AMBER | days | 1-2 kcal/mol | Specialized | Limited tools |
Decision: For lead-optimization SAR validation, OpenFE RBFE (open) or FEP+ (commercial) is the standard. For prospective discovery, MM/GBSA is a fast first-pass (3-5 kcal/mol RMSE); use FEP for top 10-50 candidates.
Decision Tree by Scenario
| Scenario | Recommended workflow |
|---|
| Rank close analogs (R-group SAR) | RBFE via OpenFE (cycle: lig1↔lig2↔lig3) |
| Cross-scaffold ranking | ABFE per ligand; or coordinated RBFE with star network |
| Lead optimization 10-50 compounds | RBFE; perturbation-graph design |
| Single ligand affinity | ABFE (no reference needed) |
| Quick first-pass on top 1k | MM/GBSA after docking |
| Novel scaffold prospective | Boltz-2 affinity + FEP confirmation on top |
| Selectivity (target vs off-target) | RBFE on both proteins; report delta-delta-G |
| Allosteric vs orthosteric | ABFE comparable; check pose stability with MD |
| Ions / metal centers | Specialized force field (ZAFF, MCPB.py); not standard FEP |
Relative Binding Free Energy (RBFE) Setup
Goal: Calculate delta-delta-G between two ligands (lig1 → lig2) in pocket.
Approach: Alchemical transformation lig1 → lig2 in both bound state (pocket + ligand + water) and unbound state (ligand + water alone). Thermodynamic cycle:
delta(delta-G_binding) = (delta-G_lig1->lig2 in pocket) - (delta-G_lig1->lig2 in solvent)
from openfe import SmallMoleculeComponent, ProteinComponent, SolventComponent
from openfe.protocols.openmm_rfe import RelativeHybridTopologyProtocol
protein = ProteinComponent.from_pdb_file('receptor.pdb')
ligA = SmallMoleculeComponent.from_sdf_file('ligand_A.sdf')
ligB = SmallMoleculeComponent.from_sdf_file('ligand_B.sdf')
solvent = SolventComponent()
protocol = RelativeHybridTopologyProtocol(
RelativeHybridTopologyProtocol.default_settings()
)
OpenFE's setup automates: mapping atoms between ligands (LOMAP), building hybrid topology, generating lambda windows, equilibration, production MD.
Lambda Window Scheduling
| Stage | Lambda values | Purpose |
|---|
| Decoupling (vdW) | 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 1.0 | Turn off ligand vdW |
| Charging (Coulomb) | 0.0, 0.25, 0.5, 0.75, 1.0 | Turn off ligand partial charges |
| Restraint (ABFE only) | 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0 | Boresch-style restraints |
Modern best practice uses 12-20 lambda windows per leg. Sampling at each window: 5-20 ns. Total simulation time per pair: 1-5 GPU-days.
REST2 Enhanced Sampling
REST2 (Replica Exchange with Solute Tempering) is the de facto standard for FEP enhanced sampling. Scales solute-solute and solute-solvent interactions; allows ligand to overcome local minima.
In FEP+, REST2 region typically includes:
- The entire ligand
- Flexible binding-site loops
- Catalytic / ionic residues with high pKa shift potential
In OpenFE, REST2 is automatically applied to the alchemical region.
MBAR/BAR Analysis
After production simulation, extract delta-G via MBAR (Multistate Bennett Acceptance Ratio) or BAR (Bennett Acceptance Ratio). MBAR uses data from all windows simultaneously; BAR uses adjacent windows.
from alchemlyb.parsing import gmx
from alchemlyb.estimators import MBAR
import pandas as pd
u_nks = []
for window in range(12):
df = gmx.extract_u_nk(f'window_{window}.xvg', T=300)
u_nks.append(df)
u_nk = pd.concat(u_nks)
mbar = MBAR().fit(u_nk)
print(f'delta-G: {mbar.delta_f_.iloc[0, -1]:.2f} +/- {mbar.d_delta_f_.iloc[0, -1]:.2f} kcal/mol')
alchemlyb is the standard analysis package for FEP results from GROMACS, AMBER, OpenMM.
Cycle Closure Analysis
Thermodynamic cycles must close (sum of edges = 0). Cycle closure error = root-mean-square error across closed cycles.
def cycle_closure(rbfe_results, cycle):
total = sum(d_g for _, _, d_g, _ in cycle)
total_var = sum(sd**2 for _, _, _, sd in cycle)
return total, total_var ** 0.5
Acceptable cycle closure: < 0.5 kcal/mol RMS. Higher indicates insufficient sampling or force-field issues.
Absolute Binding Free Energy (ABFE)
ABFE computes delta-G of binding for a single ligand (no reference compound).
Goal: Compute Kd or Ki for a single ligand prospectively.
Approach: Decouple ligand from solvated state and from pocket-bound state separately; correction terms for analytical end states.
openfe absolute-free-energy run \
--protein receptor.pdb \
--ligand ligand.sdf \
--output abfe_results/ \
--n-lambda-charge 5 \
--n-lambda-vdw 12 \
--n-lambda-restraint 7
ABFE is harder than RBFE: requires Boresch-style restraints to keep ligand near pocket during decoupling. Restraint contribution must be analytically corrected.
ABFE cost: ~3x RBFE cost per ligand.
MM/PBSA, MM/GBSA Endpoint Methods
Fast (<1 hour) alternative; lower accuracy (3-5 kcal/mol RMSE):
MMPBSA.py -i input.in -cp complex.parm7 -rp receptor.parm7 \
-lp ligand.parm7 -y trajectory.nc
Sample input:
&general
startframe = 100, endframe = 1000, interval = 10
&gb
igb = 5
&pb
istrng = 0.150
Use case: Rank order top 100 docking poses; MM/GBSA correlates ~0.5-0.7 with experimental binding; better than docking score (0.3-0.5) but worse than FEP (0.7-0.9).
Force Field Selection
| Force field | Use for | Notes |
|---|
| OPLS4 (Schrödinger) | FEP+ default | Commercial; well-tested |
| OpenFF SAGE 2.x | OpenFE default | Open-source modern |
| GAFF2 | AMBER FEP | Use for ligand only; protein FF14SB |
| GAFF | Legacy | Replaced by GAFF2 |
| CGenFF | CHARMM-style FEP | CHARMM force-field family |
| ANI-2x | Mixed QM/MM | Experimental for FEP |
| MACE-OFF | Modern ML force field | Promising for FEP, limited tooling |
For OpenFE: defaults are SAGE 2.1.0 for ligand, FF14SB for protein, TIP3P for water. Override only if benchmarking.
Per-Tool Failure Modes
Insufficient sampling
Trigger: Lambda windows simulated < 5 ns each; ligand has slow rotamer change.
Mechanism: REST2 helps but isn't a panacea; some conformational changes take 100s of ns.
Symptom: Replicates disagree by > 1 kcal/mol; cycle closure > 1 kcal/mol RMS.
Fix: Increase per-window sampling to 10-20 ns; add REST2 to additional residues; check if pose is genuinely stable.
Force-field artifacts
Trigger: Charged ligand or charged pocket residue.
Mechanism: GAFF2/SAGE may misparameterize unusual functional groups (perfluoro, charged sulfonate near Asp/Glu).
Symptom: Large RBFE error (>2 kcal/mol) for a specific transformation.
Fix: Visual inspection; check ligand topology with rdkit; consider non-bonded fix or fragment-specific parameters.
Mapping ambiguity
Trigger: Two ligands differ in scaffold (not just R-groups).
Mechanism: LOMAP atom mapping may not find good correspondence; results from ambiguous mappings unreliable.
Symptom: Mapping score low; large dummy-atom count; cycle closure errors.
Fix: Manual mapping using OpenFE's editor; or use ABFE per ligand instead of RBFE.
Restraint contribution wrong (ABFE)
Trigger: Boresch restraint applied to flexible region of ligand.
Mechanism: Analytical restraint correction assumes harmonic potential at well-defined minimum.
Symptom: ABFE differs by >3 kcal/mol from experiment systematically.
Fix: Choose Boresch restraint atoms from rigid ligand core; not flexible side chains.
MM/GBSA -- bias from entropy missing
Trigger: Comparing ligands of very different size.
Mechanism: MM/GBSA misses entropy contribution; larger ligands appear more favorable.
Symptom: Larger ligands always rank higher.
Fix: Use MM/GBSA only for within-series ranking; supplement with FEP for cross-size.
Boltz-2 affinity -- chemotype OOD
Trigger: Novel chemotype outside training distribution.
Mechanism: Boltz-2 is trained on PDBbind + ChEMBL; novel scaffolds extrapolate.
Symptom: Boltz-2 affinity and FEP affinity disagree.
Fix: Use Boltz-2 as a screen; validate top candidates with FEP. Treat Boltz-2 confidence band carefully.
Reconciliation: FEP+ vs OpenFE
| Aspect | FEP+ | OpenFE |
|---|
| Force field | OPLS4 (proprietary) | SAGE 2.1.0 (open) |
| Workflow | Schrödinger GUI | Python CLI/API |
| Atom mapping | Automated LOMAP-style | LOMAP via openmm-tools |
| Reported accuracy | 1-2 kcal/mol RMSE | Comparable; emerging benchmarks |
| Cost | Schrödinger license | Free + compute time |
| Decision | Commercial team default | Open-source / academic / cost-sensitive |
For new groups, OpenFE 1.7+ is the recommended starting point; FEP+ is the gold standard for established pharma pipelines.
Common Errors
| Symptom | Cause | Fix |
|---|
| Lambda window simulation diverges | Bad initial pose | Re-relax pose with MM minimization first |
| Cycle closure > 1 kcal/mol | Insufficient sampling | Increase per-window time; check replicate convergence |
| MBAR returns NaN | Insufficient overlap between windows | Add intermediate lambda windows |
| Restraint contribution wrong | Boresch atoms on flexible region | Choose 3 atoms on rigid ligand core |
| GROMACS REST2 setup wrong | Hot region not specified | -rest2-hot 'protein and resi 100-110' style selection |
| ABFE under-estimates by ~3 kcal/mol | Forgetting analytical correction | Apply delta-G_restraint_correction term |
| MM/GBSA rmsd doesn't match docking | Different trajectory frames | Compute MM/GBSA on MD-relaxed pose |
References
- Mey et al., Living J. Comput. Mol. Sci. 2:18378 (2020) -- alchemical free energy best practices.
- Wang et al., J. Am. Chem. Soc. 137:2695 (2015) -- FEP+ method.
- Open Free Energy (OpenFE) consortium 2023+ -- OpenFE framework. Cite the current release via the OpenFE Zenodo DOI (https://github.com/OpenFreeEnergy/openfe); the earlier "Henderson 2023 Comput Phys Commun" attribution could not be verified.
- Cournia et al., J. Chem. Inf. Model. 60:4153 (2020) -- RBFE for lead optimization.
- Aldeghi M et al -- ABFE protocols (consult current literature; the earlier "Aldeghi 2018 J Cheminform 10:43" citation could not be verified — Aldeghi's 2018 ABFE work appeared as a Methods in Molecular Biology book chapter).
- Wohlwend et al. (2025) -- Boltz-2 affinity prediction.
- Shirts & Chodera, J. Chem. Phys. 129:124105 (2008) -- MBAR.
- Bennett, J. Comput. Phys. 22:245 (1976) -- BAR.
Related Skills
- chemoinformatics/virtual-screening - Source poses for FEP input
- chemoinformatics/pose-validation - PoseBusters-validate before FEP
- chemoinformatics/conformer-generation - Generate ligand 3D for FEP setup
- chemoinformatics/molecular-standardization - Standardize ligand before FEP
- chemoinformatics/ml-docking-rescoring - Boltz-2 affinity as alternative
- chemoinformatics/qsar-modeling - Surrogate models for high-throughput
