Gaussian Augmented Plane Waves

The Gaussian augmented plane wave (GAPW) method extends GPW so that all-electron calculations and calculations with very small-core pseudopotentials become practical in CP2K. The central idea is to keep the smooth part of the density on the regular GPW grids while treating the rapidly varying density close to the nuclei with atom-centered contributions.

GAPW is useful when the core electron density matters, for example in all-electron calculations, core-level spectroscopy, magnetic properties, and some small-core pseudopotential setups. For standard valence-only pseudopotential DFT calculations, GPW is usually simpler and faster.

Activating GAPW

GAPW is activated in the QS section:

&FORCE_EVAL
  METHOD Quickstep
  &DFT
    &QS
      METHOD GAPW
    &END QS
  &END DFT
&END FORCE_EVAL

All-electron GAPW calculations also require all-electron basis sets and POTENTIAL ALL for the corresponding atomic kinds:

&KIND O
  BASIS_SET SVP-MOLOPT-GGA-ae
  POTENTIAL ALL
  LEBEDEV_GRID 110
  RADIAL_GRID 80
&END KIND

A complete tested water example is available as gapw_h2o.inp. It is intentionally small and is meant as a starting point rather than as a production-quality benchmark.

Accuracy Parameters

Several GAPW-specific tolerances control the split between soft grid-based and hard atom-centered contributions:

  • EPSFIT controls how Gaussian exponents are split into the hard and soft parts. Lowering it includes harder functions in the soft density and usually requires a larger CUTOFF.

  • EPSRHO0 controls the range used for the hard compensation density contribution.

  • EPSSVD controls the singular value decomposition tolerance used for projector matrices.

The atom-centered integration grid is controlled per kind with LEBEDEV_GRID and RADIAL_GRID. Increasing these values can improve the electron count and the accuracy of properties that depend on the near-core density, but it also increases cost.

Practical Guidance

When setting up a GAPW calculation:

  • Use an all-electron basis set for POTENTIAL ALL, or a basis set designed for the chosen small-core pseudopotential.

  • Inspect the electron count printed by CP2K after SCF convergence. It is a useful diagnostic for the quality of the hard/soft density split.

  • Tighten EPSFIT, EPSRHO0, EPSSVD, and the atomic grids only as much as needed for the target property.

  • Increase the density CUTOFF when harder Gaussian exponents are included in the soft density.

  • Prefer GPW when the calculation does not need all-electron or near-core accuracy.

See Also