From: Alexandre Suman de Araujo (
Date: Fri May 03 2013 - 15:40:41 CDT

Hi all

I'm modeling a small molecule using FFTK plugin in VMD version 1.9.2a14.

I have some questions about the charge optimization procedure shown in
the screencasts tutorial. There, after QM calculation of the interaction
energy between water molecules and hydrogen bond donor/acceptor groups,
it goes to the charge optimization step.

Basically, this step consists in loading of structure information,
loading of force field parameters already obtained, guess of charge
constraints and values, loading of QM log files and, finally, the
adjustment of the partial charges using an optimization procedure
(simulated annealing or downhill).

In the "Results" tab, there is an option to set the calculated charges
as initial charges to perform, in my acknowledgment, another
optimization calculation. However, after this new optimization the
charges are very different from the first ones obtained and used as new
initial charges. The partial charges values only converge after some
cycles of optimization, and my question is about the obtained results:

  - If I compare these converged charges with Mulliken charges from SP
calculation, I found strong disagreements between them. Including signal
inversion of some charges (QM = -1.4, Final optimized partial charges =
- Another inconsistency are positive partial charges of hydrogen bond
acceptors and negative of donors.

The procedure to adjust the charges in several cycles is the correct
approach? If yes, where I am doing mistakes? If no, how should I proceed?

Best regards

Prof. Dr. Alexandre Suman de Araujo
Departamento de Física
Instituto de Biociências, Letras e Ciências Exatas - IBILCE
Universidade Estadual Paulista
Phone: +55 (17) 3221-2566/3221-2240