[CP2K-user] [CP2K:21091] Re: Self-consistent Hirshfeld: How were ref atoms made, and can I extract them?

[sobereva at sina.com]

Dear Alex,

Yes, this is a well-known shortcoming of Hirshfeld-I. The MBIS charge (J. Chem. Theory Comput. 2016, 12, 3894−3912, J. Phys. Chem. A 2018, 122, 17, 4219–4245) overcomes this problem, as it doesn't need radial denisty of different charged states of involved elements. However, MBIS also has some issues, as partially revealed in the comparison in my review about atomic charges (DOI: 10.1002/9783527843435.ch6), and it doesn't work reasonably for some anions.


--------------------------------

Best regards,
 
Tian Lu
----- Original Message -----
From: Alexander Davis <alexlovesmolecules at gmail.com>
To: cp2k <cp2k at googlegroups.com>
Subject: [CP2K:21090] Re: Self-consistent Hirshfeld: How were ref atoms made, and can I extract them?
Date: 2025-01-30 13:06

Hi Prof Lu,
It is great news that it is possible to extract the reference ion densities used by MultiWFN for Hirshfield-I calculation. I am an appreciative MultiWFN user—in fact I use it in my own Hirshfeld-I implementation to create reference atom cube files on the appropriate grid—and with the reference atom densities in hand, it seems that I could use MultiWFN for the whole charge calculation, since I will be able to use those .rad files to reconstruct the weight function after the charge calculation.
However, it seems that the MultiWFN reference atoms have the issue that I am trying to avoid: unattached electrons in the anions. I plotted the radial densities of the nitrogen references found in "examples/atmrad":

As expected and desired, the cation densities get wider as electrons are added. However, the anion densities stay exactly the same all the way up to past an angstrom from the nucleus. The additional electron density all ends up out past the covalent radius of nitrogen.
The reason for this is that N- and N2- are artificial. The electron affinity of nitrogen is near zero. N- is not really stable, or perhaps barely stable, and N2- is certainly not stable. As a result, the electrons are unattached in the reference densities.
This issue was discussed extensively by Vanpoucke, Bultinck, and Van Driessche in their paper "Extending Hirshfeld-I to Bulk and Periodic Materials", and generated considerable discussion: a comment by Thomas Manz, followed by a reply to that comment by the original authors. It is not a simple issue with a neat solution, and the main reason I am looking around at Hirshfeld-I implementations other than my own is that I'm hoping that someone else has worked out a satisfactory alternative.
In any case thanks for the information, and I love MultiWFN. Wishing you the best,Alex

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