On Mon, May 12, 2014, Jason Swails wrote:
>
> Yuck. I would guess that you're right in that this is a limitation of
> using gas-phase structures in charge derivation. (I believe such
> zwitterions are only stable in solution.)
>
> A relatively simple alternative I can see would be to use some kind of
> implicit solvent model (either GB in sander or PCM/COSMO in Gaussian)
> when you minimize before using the standard HF/6-31G* Merz-Kollman
> scheme to get the partial charges.
>
> Of course the real issue is working this into a reliable, automated
> workflow...
Just to add my two cents here: in some sense the problem arises from Chris
Bayly's decision to define the AM1-BCC charge model in terms of minimized
AM1 structures. This makes good sense for many systems, and makes the
resulting charges not depend on details of some initial geometry. But it
clearly fails in circumstances like the ones defined here.
If one is clear about making a change to the original procedure, skipping the
geometry optimization step (say for certainly types of molecule)
would seem like an attractive idea.
...dac
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Received on Mon May 12 2014 - 05:30:03 PDT
