relax

name	relax
description	Prepare VASP geometry-relaxation input tasks from a user-provided structure and essential DFT settings. Use when the user needs ionic or cell-coupled relaxation and requires explicit ISIF-driven relaxation intent mapping, INCAR generation, and POTCAR mapping instructions.
compatibility	Requires a user-provided structure 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 Relaxation (Subskill)

Scope

This skill prepares relaxation tasks only. It generates POSCAR, INCAR, optional KPOINTS, and POTCAR mapping instructions. It does not submit or execute jobs — hand off to dpdisp-submit.

Relaxation intent is mandatory

Before assigning ISIF, classify user intent. The canonical VASP wiki ISIF table:

ISIF	Positions	Cell shape	Cell volume	Typical use
2	yes	no	no	Ion-only relax at fixed cell (defects, surfaces with frozen vectors)
3	yes	yes	yes	Full cell+ion relax — bulk equilibrium
4	yes	yes	no	Cell-shape+ion relax at fixed volume (non-cubic phase search)
5	yes	yes	no	Like 4 but no stress check
6	no	yes	yes	Cell shape + volume, atoms frozen in direct coordinates
7	no	no	yes	Volume-only (isotropic scan, rarely used directly)

If intent is ambiguous, ask the user for clarification.

Must provide

• structure input (POSCAR)
• ENCUT (see "ENCUT for relaxation" below)
• ISMEAR / SIGMA (use the static/SCF table from dft-vasp/static)
• relaxation controls: IBRION, NSW, EDIFFG, ISIF
• k-point policy (KSPACING or explicit KPOINTS)
• POTCAR mapping per element

Usually should be explicit

• EDIFF (1E-6 for NEP labeling seed structures, 1E-5 otherwise)
• NELM (60 default; raise to 100 if SCF struggles)
• PREC = Accurate (required when any cell-DOF is relaxed)
• POTIM (see "Algorithm selection")
• LREAL = .FALSE. (small cells) or Auto (> 30 atoms)
• ISPIN / MAGMOM when magnetic
• IVDW when vdW is physically relevant

ENCUT for relaxation

• Cell-DOF relaxations (ISIF ∈ {3,4,5,6,7,8}) must use ENCUT = 1.3 × max(ENMAX) to suppress Pulay stress. This is a physical requirement, not a convention: Pulay stress biases the cell toward smaller volumes when the plane-wave basis set adapts to a changing cell. 30% headroom is the community standard that makes this bias smaller than typical EDIFFG.
• Ion-only relaxations (ISIF = 2) can use ENCUT = max(ENMAX).

Algorithm selection (IBRION / POTIM)

Based on VASP wiki IBRION:

IBRION	Algorithm	When to pick	POTIM
2	Conjugate gradient	Robust default. Pick first.	0.5
1	RMM-DIIS quasi-Newton	> 20 DOF and already near the minimum	0.5
3	Damped MD	Large systems far from the minimum	0.1

Rule of thumb: start with IBRION=2, only switch to 1 if CG stalls after ~10 NSW steps, and only use 3 when a structure came from a non-VASP source (force field, cif with bad geometry).

EDIFFG convention

• Negative value → force criterion in eV/Å (recommended). -1E-2 for quick screening, -1E-3 for production, -5E-4 for phonon seeds.
• Positive value → total energy change in eV. Weaker physical meaning, discouraged unless the system has phonon-soft modes.

Concrete example — fcc Si full cell+ion relax

Anchored on VASP wiki Fcc Si. The 1-atom primitive cell has no internal ion DOF, so ISIF=3 only relaxes the lattice constant — this is the minimal useful relaxation test.

Task directory:

Si_fcc_relax/
├── POSCAR
├── INCAR
├── KPOINTS
└── POTCAR            # cat <POTPAW>/Si/POTCAR

POSCAR (start a = 3.9 Å — the VASP wiki EOS scan starting point):

fcc Si
  3.9
    0.5  0.5  0.0
    0.0  0.5  0.5
    0.5  0.0  0.5
Si
  1
Direct
  0.0  0.0  0.0

INCAR:

SYSTEM = fcc Si relax
ISTART = 0
ICHARG = 2
ENCUT  = 320          # 1.3 × ENMAX(Si) = 1.3 × 245 ≈ 320 (Pulay headroom)
PREC   = Accurate
LREAL  = .FALSE.
EDIFF  = 1E-6
EDIFFG = -1E-3        # force threshold 1 meV/Å
NELM   = 60
ISMEAR = 0
SIGMA  = 0.05
IBRION = 2            # CG
ISIF   = 3            # cell + ion (ion is symmetry-locked for Si primitive)
NSW    = 40
POTIM  = 0.5
LWAVE  = .FALSE.
LCHARG = .FALSE.

KPOINTS: same 11×11×11 Γ-centered mesh as the static SCF example.

Physical sanity checks after the run

1. grep 'reached required accuracy' OUTCAR — one line means converged.
2. grep ' volume of cell' OUTCAR | tail -1 — for this fcc 1-atom-primitive Si test (the VASP wiki EOS toy model, not real diamond Si), PBE equilibrium volume is ≈ 14.5 ų/atom, corresponding to a ≈ 3.87 Å in the fcc-primitive convention. Deviation > 0.3 ų signals ENCUT too low or k-mesh too coarse. (Real diamond Si with 2 atoms/primitive has PBE volume ≈ 20.4 ų/atom and a_conv ≈ 5.47 Å — a different POSCAR.)
3. grep 'external pressure' OUTCAR | tail -1 — residual |pressure| < 1 kBar.
4. grep 'TOTEN' OUTCAR | tail -1 — final energy lower than the starting SCF energy by ~3–10 meV/atom (the Pulay contraction).
5. Pulay check: rerun the final geometry as a static SCF at the same ENCUT. If the static TOTEN differs from the relax TOTEN by > 1 meV/atom, the relax ENCUT was still too low — raise it and repeat.

Slab / low-dimensional policy

• For slabs, never use ISIF=3: the vacuum axis collapses.
• Use ISIF=2 with selective dynamics in POSCAR to freeze bottom layers, or ISIF=4 if you want to allow in-plane lattice rebuild while keeping the vacuum thickness.
• For 2D heterobilayers (e.g. the Bi/MoS₂ case used by the GPUMD skill family), LDIPOL=.TRUE. + IDIPOL=3 is recommended to remove spurious dipole interaction across the vacuum.

Known build traps

• Non-collinear/SOC (LSORBIT=.TRUE.) requires a non-NGXhalf build. The standard vasp_std will crash — see dft-vasp/static for details.
• IBRION=5/6/7/8 (DFPT phonons) require additional settings — they are out of scope for this relaxation skill.

Expected output

1. task directory with generated input files
2. explicit ISIF selection rationale (quote the table row)
3. ENCUT + Pulay rationale
4. physical sanity check checklist
5. unresolved choices (functional, vdW, spin)
6. handoff note to dpdisp-submit