Skip to content

BMD Compute Advanced Options

Jump to bottom
Lee Burton edited this page Sep 26, 2026 · 3 revisions

BMD Compute Advanced Options

Authority. BMD Compute is authoritative for how BMD VASP calculations are generated and executed. BMDex supplies curated supporting data, evidence and tools. Scientific validation and adoption are separate human review.

BMD Compute's advanced options (modifiers) change a base calculation for physics that the base settings leave out. This page explains the concepts behind each option and when it matters. Which stages and levels of theory accept each option is shown by the BMD Compute interface, which is authoritative.

This is the last of four pages, after VASP — fundamentals, BMD Compute Workflows and BMD Compute INCARs. Each option here will be shown as a change (a delta) from the base INCAR files on the INCARs page.

Reference deltas are not published yet. For each option, the exact INCAR changes will be added here only once they have been generated by BMD Compute. Until then, use the inputs BMD Compute generates for your own calculation, and do not copy settings from other wiki pages.

How to read an option

For each option, this page will show:

  • Concept — the physics or numerical issue it addresses.
  • When it matters — the kinds of materials or questions where you need it.
  • Reference delta — the lines BMD Compute adds (+) or removes (-) compared with the base INCAR. Pending.

Spin polarisation

Concept. The base reference calculations use ISPIN = 1, which treats spin-up and spin-down electrons identically. A spin-polarised calculation lets the two spin channels differ, so the calculation can describe magnetic moments. It needs starting magnetic moments for each atom, and the result can depend on them.

When it matters. Materials that may be magnetic: many compounds containing transition metals or rare earths, open-shell defects and radicals. It roughly doubles the electronic work, so it is not used for clearly non-magnetic materials like Si.

Reference delta. Pending.

DFT+U

Concept. Standard semilocal functionals such as PBE over-delocalise strongly localised d and f electrons. DFT+U adds an on-site correction, set by a parameter U for chosen elements, that penalises fractional occupation of those orbitals. The value of U depends on the element and its chemical environment, and energies computed with different U values cannot be compared directly.

When it matters. Compounds of transition metals and rare earths with localised d or f states, often oxides and fluorides, where plain PBE gives qualitatively wrong band gaps, magnetism or oxidation states. It does not apply to simple semiconductors such as Si, and the base reference INCAR files contain no DFT+U tags.

Reference delta. Pending (needs a material to which a U applies).

van der Waals (dispersion) correction

Concept. Semilocal functionals miss most of the long-range dispersion (van der Waals) attraction between atoms that are not chemically bonded. Grimme's DFT-D3 family of corrections adds this as an extra, pairwise energy term; D3(BJ) is the variant with Becke–Johnson damping at short range.

When it matters. Layered materials (for example graphite or transition-metal dichalcogenides), molecular crystals, and molecules adsorbed on surfaces, where the interlayer or intermolecular spacing is set by dispersion. Without it, relaxed interlayer distances can be far too large.

Reference delta. Pending.

Gamma-only k-points

Concept. Sampling the Brillouin zone at the single Γ point instead of a mesh. For very large cells (big supercells, amorphous structures, molecules in a box) the Brillouin zone is small and Γ alone can be adequate.

When it matters. Large systems where a full mesh would be unnecessary and expensive. It is not suitable for small cells such as bulk Si, and it cannot be used for a band structure, which needs a path of k-points.

Reference delta. Pending.

Ions-only relaxation

Concept. A geometry optimisation that moves the atoms but keeps the cell shape and volume fixed. In VASP this corresponds to relaxing ionic positions only, instead of positions, shape and volume as in the base Geometry Optimisation (ISIF = 3).

When it matters. When the cell must stay fixed, for example a defect or surface calculation in a supercell whose lattice parameters were already determined.

Reference delta. Pending.

Spin–orbit coupling (SOC)

Under validation. Automatic SOC methodology in BMD Compute is still under validation, and this section is intentionally incomplete. Do not treat SOC settings from any wiki page as BMD Compute policy.

Concept. Spin–orbit coupling links an electron's spin to its orbital motion. It is strongest for heavy elements and can split and shift bands noticeably. Including it requires a non-collinear calculation, which is considerably more expensive.

Reference delta. Not documented until validation is complete.

Related pages

Clone this wiki locally