Bader's atom-in-molecule analysis based on GAPW total electron density, is it possible?

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Oct 15, 2021, 4:17:04 PMOct 15
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Dear CP2K developers, 

Is it possible to use GAPW calculated total electron density for Bader's atom-in-molecule analysis, such as finding the bond critical point and calculate the bond electron density? 

I noticed that for GAPW, because the total electron density is partitioned into three parts: smooth all-space part, hard atomic part, soft atomic part, and the total electron density is equal to: smooth all-space part + hard atomic part - soft atomic part.

As a consequence of the partition, the total electron density cube, has negative electron density which is from the (minus) soft atomic part. 

My question is, if we want to use the total electron density by GAPW, which contains the negative electron density, for Bader's method, is it possible or not possible. 

I noticed long time ago, Axel mentioned this that M. Krack implemented GAPW for Bader's AIM, and had some issue, could you please give me some more details about what these issues were? Thanks!

Here is the original email, and Alex mentioned about M. Krack's effort on AIM,

Axel's comments: 
not quite. for AIM you need the _complete_ wavefunction, whereas
cp2k usually only outputs the valence part (even with *PAW) and
that can have a significant impact on the validity and accuracy
of the results. so you have to first add the core wavefunction
part back that is subtracted out during pseudopotential generation
or through the PAW procedure.

matthias krack once implemented a feature that you can do this
from GPAW, but it turned out to be extremely slow and had artefacts
that threw the bader code from the group in texas off.

i have been toying with the idea of constructing density files
to add to the existing valence density after the fact, by taking
the difference from a single atoms all-electron and pseudopotential
calculation, and then adding them to the valence density cube.
but that scheme only worked for me to some degree for s-electrons.

Thank you very much!!


Fangyong Yan

Oct 16, 2021, 9:41:55 AMOct 16
Dear Dr. Matthias Krack and Professor Hutter,

For the GAPW electron density, when I print the total electron density, there are negative electron density, how does it happen? 

The negative electron density should come from the soft atomic part, but since both soft atomic part and hard atomic part uses the same grid, so on each grid point, the electron density should be positive, could you please explain the negative electron density? 



Print the total electronic density in the case of a GAPW run. This keyword has only an effect, if PAW atoms are present. The default is to print only the soft part of the electronic density and to ignore the hard part. NOTE: The total density in real space might exhibit unphysical features like spikes due to the finite and thus truncated g vector expansion

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Fangyong Yan

Oct 18, 2021, 11:59:54 AMOct 18
I used the square of the wavefunction and obtain the correct electron density from the GAPW calculation. Then I used the electron density to calculate the Bader's charge using Henkelman's code, and it gives the Bader charge for a water molecule. 

So it is possible to calculate the Bader's charge using GAPW, although it is not trivial. 


Below is the Henkelman's Taxas group code, 

     #         X           Y           Z       CHARGE      MIN DIST   ATOMIC VOL


    1    0.000000    0.000000   -0.147753    8.988720     1.307413   132.629633

    2    0.000000   -1.438202    0.996119    0.474505     0.282764    25.592810

    3    0.000000    1.438202    0.996119    0.474505     0.282764    25.592810


    VACUUM CHARGE:               0.0669

    VACUUM VOLUME:           27457.0941

    NUMBER OF ELECTRONS:        10.0046

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