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87 lines
3.1 KiB
Markdown
87 lines
3.1 KiB
Markdown
# Gaussian Augmented Plane Waves
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The Gaussian augmented plane wave ({term}`GAPW`) method extends GPW so that all-electron
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calculations and calculations with very small-core pseudopotentials become practical in CP2K. The
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central idea is to keep the smooth part of the density on the regular GPW grids while treating the
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rapidly varying density close to the nuclei with atom-centered contributions.
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GAPW is useful when the core electron density matters, for example in all-electron calculations,
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core-level spectroscopy, magnetic properties, and some small-core pseudopotential setups. For
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standard valence-only pseudopotential DFT calculations, GPW is usually simpler and faster.
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## Activating GAPW
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GAPW is activated in the [QS](#CP2K_INPUT.FORCE_EVAL.DFT.QS) section:
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```text
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&FORCE_EVAL
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METHOD Quickstep
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&DFT
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&QS
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METHOD GAPW
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&END QS
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&END DFT
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&END FORCE_EVAL
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```
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All-electron GAPW calculations also require all-electron basis sets and `POTENTIAL ALL` for the
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corresponding atomic kinds:
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```text
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&KIND O
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BASIS_SET SVP-MOLOPT-GGA-ae
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POTENTIAL ALL
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LEBEDEV_GRID 110
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RADIAL_GRID 80
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&END KIND
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```
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A complete tested water example is available as [](gapw_h2o.inp). It is intentionally small and is
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meant as a starting point rather than as a production-quality benchmark.
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## Accuracy Parameters
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Several GAPW-specific tolerances control the split between soft grid-based and hard atom-centered
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contributions:
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- [EPSFIT](#CP2K_INPUT.FORCE_EVAL.DFT.QS.EPSFIT) controls how Gaussian exponents are split into the
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hard and soft parts. Lowering it includes harder functions in the soft density and usually
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requires a larger [CUTOFF](#CP2K_INPUT.FORCE_EVAL.DFT.MGRID.CUTOFF).
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- [EPSRHO0](#CP2K_INPUT.FORCE_EVAL.DFT.QS.EPSRHO0) controls the range used for the hard compensation
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density contribution.
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- [EPSSVD](#CP2K_INPUT.FORCE_EVAL.DFT.QS.EPSSVD) controls the singular value decomposition tolerance
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used for projector matrices.
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The atom-centered integration grid is controlled per kind with
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[LEBEDEV_GRID](#CP2K_INPUT.FORCE_EVAL.SUBSYS.KIND.LEBEDEV_GRID) and
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[RADIAL_GRID](#CP2K_INPUT.FORCE_EVAL.SUBSYS.KIND.RADIAL_GRID). Increasing these values can improve
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the electron count and the accuracy of properties that depend on the near-core density, but it also
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increases cost.
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## Practical Guidance
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When setting up a GAPW calculation:
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- Use an all-electron basis set for `POTENTIAL ALL`, or a basis set designed for the chosen
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small-core pseudopotential.
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- Inspect the electron count printed by CP2K after SCF convergence. It is a useful diagnostic for
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the quality of the hard/soft density split.
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- Tighten `EPSFIT`, `EPSRHO0`, `EPSSVD`, and the atomic grids only as much as needed for the target
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property.
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- Increase the density [CUTOFF](#CP2K_INPUT.FORCE_EVAL.DFT.MGRID.CUTOFF) when harder Gaussian
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exponents are included in the soft density.
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- Prefer GPW when the calculation does not need all-electron or near-core accuracy.
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## See Also
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- [](#Lippert1999)
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- [](#Krack2000)
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- [](#Iannuzzi2026)
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```{youtube} L0hKLjvjIFU
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---
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url_parameters: ?start=4
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align: center
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privacy_mode:
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---
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```
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