[CP2K-user] [CP2K:21726] RESP charges in a small cell - Fitting procedure wrong?

[Andreas Döll]

Hello,

I am trying to calculate the RESP charges for a quasi 2d COF layer 
structure with periodic boundary conditions. I will be using them as an 
input for MD simulations. I have performed geometric optimization and the 
cell optimization sucessfully and the resulting structure looks reasonable. 
Afterwards I do a single point calculation using the CAMB3LYP basis set and 
fit my RESP charges to the ESP. However, I noticed that the charges I 
obtain are quite unreasonable, even though mt RMS of the RESP fit is 10^-6, 
and RRMS is 10^-2

Although the charges I obtained on the Nitrogen and Sulfur are somewhat 
expected, lots of carbons which are part of benzene rings have large 
charges magnitude wise, almost in an alternating fashion even far away from 
any N/S! (Ex.: Atom 1-6 is a central benzene ring, where 3 other benzene 
rings are attached to it, I would expect a low magnitude of the charges: or 
atoms 17/18 which are part of a benzene ring but have alternating charges 
of +0.31 and -0.29 respectively). All of this leads me to doubt my charge 
fitting procedure.

Enforcing symmetry just leads to worse results and even restraints did not 
help, as restraining the carbons leads to bad charges on N/S. 
I think the problem comes from the small z direction of my unit cell, which 
leads to an ESP which is strongly influenced by the periodic image and 
results in a weird RESP fit. But since a singular COF layer is mostly flat, 
a small z direction is expected. Are these charges a feature, not a bug due 
to the influence of the other layers in the triclinic cell?

Any advice or help on this matter would be appreciated! Is it maybe some 
parameter issue I am missing or something different? 

I have attached the xyz file for the structure, as well as the input and 
output file. Thank You in advance.

Sincerely, 
Andreas Doell

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