band

name	band
description	Prepare VASP band-structure workflow inputs from existing SCF context and user-specified band-path settings. Use when the user requests electronic band-structure calculations and needs explicit prerequisite checks, line-mode KPOINTS path setup, and stage-specific INCAR preparation.
compatibility	Requires prerequisite SCF context and valid VASP pseudopotential resources/license in the target environment.
catalog-hidden	true
license	GPL-3.0-only
metadata	{"author":"qqgu","version":"0.2.0","repository":"https://vasp.at/"}

VASP Band Preparation (Subskill)

Scope

This skill prepares band-structure-stage input tasks only. It verifies prerequisite SCF context, generates line-mode KPOINTS, and prepares a stage-appropriate INCAR. It does not submit or execute jobs.

Two-step workflow (canonical)

Per VASP wiki Fcc_Si_bandstructure:

1. Step 1 — SCF with LCHARG = .TRUE. on a regular Monkhorst–Pack / Γ-centered mesh (done by dft-vasp/static).
2. Step 2 — Band (this skill): fix the charge density (ICHARG = 11), sample along a high-symmetry path using line-mode KPOINTS, and use Gaussian smearing (ISMEAR = 0) — not tetrahedron, because line-mode meshes are not suitable for tetrahedron integration.

VASP wiki: "This calculation needs a converged charge density as input (ICHARG=11)."

Prerequisites (hard)

• CHGCAR (non-empty) from a prior SCF/DOS run
• same POSCAR, POTCAR, ENCUT, PREC, GGA as the prerequisite SCF
• the prior SCF ran with LCHARG = .TRUE.

If any of these is missing, stop and ask.

Must provide

• source SCF context path (verify the four items above)
• crystal symmetry / Bravais lattice (to pick the canonical k-path)
• number of points per line segment (typical: 20–40)
• band intent (plain band, projected band, fat-band)

Usually should be explicit

Tag	Role	Recommended
ICHARG	charge handling	11
ISMEAR	k-sampling method	0 (Gaussian, SIGMA = 0.05)
LORBIT	projection mode	11 for fat/projected bands, omit for plain
NBANDS	extra bands	≥ 1.3 × occupied to resolve conduction manifold
LWAVE, LCHARG	do not overwrite	.FALSE., .FALSE.

K-path selection

• Face-centered cubic (Si, Ge, C, GaAs, NaCl, ...): L – Γ – X – U | K – Γ.
• Body-centered cubic (Fe, Na): Γ – H – N – Γ – P.
• Hexagonal (MoS₂, graphene): Γ – M – K – Γ.
• Generic: use sumo-kgen or seekpath (or pymatgen-structure's high-symmetry helper) — never guess coordinates.

Concrete example — fcc Si band structure

Line-mode KPOINTS exactly as in VASP wiki Fcc_Si_bandstructure (L – Γ – X – U, then K – Γ, 20 points per segment):

k-points for bandstructure L-G-X-U K-G
 20
line
reciprocal
  0.50000  0.50000  0.50000    1    ! L
  0.00000  0.00000  0.00000    1    ! Gamma

  0.00000  0.00000  0.00000    1    ! Gamma
  0.00000  0.50000  0.50000    1    ! X

  0.00000  0.50000  0.50000    1    ! X
  0.25000  0.62500  0.62500    1    ! U

  0.37500  0.75000  0.37500    1    ! K
  0.00000  0.00000  0.00000    1    ! Gamma

INCAR:

SYSTEM = fcc Si band
ISTART = 1
ICHARG = 11
ENCUT  = 320
PREC   = Accurate
LREAL  = .FALSE.
EDIFF  = 1E-6
NELM   = 60
ISMEAR = 0
SIGMA  = 0.05
LORBIT = 11
LWAVE  = .FALSE.
LCHARG = .FALSE.

Physical sanity checks after the run

1. grep -c 'BZINTS' OUTCAR — should be 0 (line-mode k-points must not trigger Brillouin-zone integration warnings).
2. ls EIGENVAL — file must exist and be larger than POSCAR.
3. head -6 EIGENVAL — line 6 reports NELECT NKPTS NBANDS (for the fcc Si 4-segment × 20-point example: 4 80 8, i.e. 4 valence electrons, 80 k-points along the path, 8 bands auto-picked by VASP).
4. Plot EIGENVAL (or vaspkit 211, sumo-bandplot): verify connectivity — the bands must follow the path without gaps in the k-axis.
5. For fcc 1-atom Si (metallic by construction), the Fermi level lies inside a band — there is no gap. If you want the textbook diamond-Si indirect gap (Γ → 0.85 X, ~0.7 eV in PBE), use a 2-atom diamond POSCAR, not the 1-atom fcc cell.

Known build traps

• ICHARG = 11 with ISYM ≠ 0 on a low-symmetry path sometimes symmetry-reduces k-points you wanted to sample. Set ISYM = 0 if the resulting NKPTS is smaller than the number of requested line points.
• Hybrid functional band structures (LHFCALC = .TRUE.) cannot use ICHARG = 11 line-mode directly — use the zero-weight-KPOINTS trick or LOPTICS/wannier90 path.

Expected output

1. band-stage task directory with generated INCAR, line-mode KPOINTS, staged CHGCAR from the prerequisite SCF
2. generated k-path summary (list segments, labels, total NKPTS)
3. prerequisite check summary (SCF path, CHGCAR size, k-path convention)
4. settings summary (ISMEAR rationale: Gaussian not tetrahedron)
5. physical sanity check checklist
6. handoff note to dpdisp-submit