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

[Natalie Holzwarth]

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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