[CP2K-user] [CP2K:13128] How to understand/tune parameters for GAPW calculations

[hut... at chem.uzh.ch]

Hi

The GAPW method in CP2K follows the basic idea of PAW by Peter Bloechl,
but has also some special features.

In the current implementation in CP2K

- there is only one augmentation charge, meaning hard and soft augmentation
  charge from the original scheme are the same. This allows to drop all the
  additional three-center correction terms in the Coulomb part. It also means
  that the minimal cutoff needed in the soft density is related to this charge,
  typically about 200 Ry. 
  Bloechl's 'local potential' is not needed in this scheme.

- There are no frozen electrons/states. All electrons explicitly in the calculation
  are optimized. However, you can use GAPW also with pseudopotentials, as well as
  in mixed calculations with all-electron atoms and pseudopotential atoms.
  Atoms with "POTENTIAL ALL" need a corresponding all-electron basis set, atoms with
  a pseudopotential a corresponding pp-basis set.

- The basis sets used to expand the soft/hard densities are derived from the input
  orbital basis primitive functions. Functions with large exponents are in the hard set,
  functions with small exponents are in the soft set. The important input variable to
  control this are
  QS / EPSFIT  and  QS / EPSISO

  EPSFIT default is 10^-4. A larger value enlarges the PAW radius and more primitive are 
  considered in the hard set.

  EPSISO is used to determine if a primitive is completely contained in the PAW sphere.
  Such functions are their own projectors and are handled separately.

- There are many more input variables that allow to fine tune the calculations, but 
  experience shows that most of them are hardly ever used. Besides the ones mentioned above,
  I find it necessary sometimes to tighten the default values for the one-center integrations.
  This is specified in the KIND section for each atom using
  KIND / LEBEDEV_GRID  and KIND / RADIAL_GRID

I hope this helps to get started.

regards

Juerg Hutter  


--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Institut für Chemie C                FAX   : ++41 44 635 6838
Universität Zürich                   E-mail: hut... at chem.uzh.ch
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
---------------------------------------------------------------

-----cp... at googlegroups.com wrote: -----
To: "cp2k" <cp... at googlegroups.com>
From: "Natalie Holzwarth" 
Sent by: cp... at googlegroups.com
Date: 04/14/2020 05:16PM
Subject: [CP2K:13128] How to understand/tune parameters for GAPW calculations

Dear cp2k developers,
      We are very much looking forward to possibly using CP2K  particularly using the GAPW scheme to extend our calculations which up to now have been performed using ABINIT and QUANTUM ESPRESSO using PAW.  We have tried a number of your tutorials and test inputs, but are have a hard time understanding how to properly pick the parameters.   Is there perhaps a writeup somewhere that describes the detailed implementation or perhaps a list of the main subroutines that a user should study?   Or perhaps someone might have time to answer some the following silly questions.
1.  These questions are based on the notion that we should use the all-electron bases since these would allow n^1 to be represented analogously to Blochl's PAW formulation.   Is this the correct way to think about it?
2.   According the paper by Lippert et al (1999), the soft wavefunctions and densities are chosen by eliminating the Gaussian basis primitives with too large an exponent.   Which of the input parameters determines the soft basis parameters?
3.   In addition to the soft density, Blochl's PAW scheme has a local pseudo potential.    I am not sure what is the analogous local pseudo potential for GAPW and which parameter controls it. I have assumed that we should use "ALL" potentials in the input files.   Is this correct?  
4.   Some of the examples use the following parameters which seem to be important, but I am not sure how to choose them

MAX_RAD_LOCAL
ALPHA0_H
HARD_EXP_RADIUS

5.   Is there a provision/recommendation to use the frozen core approximation and if so, what parameters control that?

Thanks in advance for any suggestions you can provide and much gratitude for sharing and maintaining the cp2k software package.

Sincerely,   Natalie Holzwarth
  
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