[CP2K-user] Hirshfeld-I charges with higher precision

[Nicklas Österbacka]

Hi!

I am planning a project on constructing a machine learning potential based 
on CP2K calculations, and as part of this I need access to dependable 
partial charges. Self-consistent Hirshfeld with SHAPE_FUNCTION DENSITY 
seems to be the best option in CP2K as I would like to avoid 
post-processing of potentially hundreds of gigabytes of data.

Digging through the source code, it seems that the 
*comp_hirshfeld_i_charges* routine proceeds until the total charge residual 
is less than 1e-2. I am not sure this is enough to train a reliable machine 
learning model, however, so I would like to change it to something stricter 
to be on the safe side. Is this safe to do, or could that lead to e.g. 
numerical instabilities? Changing it to 1e-4 or 1e-5 would probably be 
enough for my purposes.

All the best,
Nicklas
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