[CP2K-user] [CP2K:21090] Self-consistent Hirshfeld: How were ref atoms made, and can I extract them?

[Jürg Hutter]

Hi

I implemented the Hirshfeld charges some (long) time ago. I just followed the original
paper on SC charges. The reference densities are calculated on the fly with the atomic
code (as for the initial guess). The shape function applies to all density (choice from
convenience, nothing deep). I'm not following the literature on this and our implementation
is probably far behind state of the art. Nothing has been published regarding details
of this implementation.

regards
JH

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Alexander Davis <alexlovesmolecules at gmail.com>
Sent: Wednesday, January 29, 2025 3:11 AM
To: cp2k
Subject: [CP2K:21087] Self-consistent Hirshfeld: How were ref atoms made, and can I extract them?

Hi,

I have been calculating self-consistent Hirshfeld charges using my own post-processing script, but I am thinking of switching to the CP2K implementation. However, there are two questions I would need answered.

One question is how it handles a well-known issue with this method, which is that it obtains the shape function from the electron densities of free ions, but sometimes these ions don't really exist. This is why my implementation is unsatisfactory: I have references for N- and N2-, but the electrons are not really attached, so the reference densities are too wide. Every self-consistent Hirshfeld implementation as to handle this somehow, I would like to know how CP2K handled it. The documentation says "This scales only the full shape function, not the added charge as in the original scheme", which is suggestive of a solution, but where can I read more about it? If the details are in one of the publications, I missed it.

My second question is whether I save the reference atoms as cube files, or even better, the actual weight function used for integration (which is a(r)/(a(r)+b(r)), where a and b are the reference atom densities). It's a quirk of my application, I need precisely the weight function that was used to calculate the charges.

Thanks,
Alex

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