VMD-L Mailing List

From: JC Gumbart (gumbart_at_physics.gatech.edu)
Date: Tue Jun 09 2020 - 12:33:39 CDT

FFTK is not using Mulliken charges. It’s trying to optimize the interaction energy with water molecules. As much as we might wish otherwise, FFTK can’t be treated as a black box. Look at the QM output - are all water molecules finding a minimum at a reasonable distance? Are the interaction energies for the optimized charges in good agreement between QM and MM? etc.

Best,
JC

> On Jun 9, 2020, at 12:15 AM, Daniel Fellner <dfel694_at_aucklanduni.ac.nz> wrote:
>
> Hi all,
>
> I'm having some difficulty deriving appropriate partial charges. The charges don't seem to match chemical intuition and differ significantly from the CGenFF-assigned charges (when not specifically excluded) for even low penalty atom types.
>
> Upon reading the Gaussian output files, I actually only find Mulliken charges in the output. I was under the impression that Mulliken charges are not suitable for such uses. Is the Gaussian input script writer perhaps not providing the correct keywords for my version of Gaussian (09-D.01)?
>
> As an example, the input line for the MP2 calculation reads:
>
> # MP2/6-31G* SCF=Tight Density=Current
>
>
> Daniel Fellner BSc(Hons)
> PhD Candidate
> School of Chemical Sciences
> University of Auckland
> Ph +64211605326