AMBER Archive (2008)

Subject: RE: AMBER: BCC charges

From: Ross Walker (ross_at_rosswalker.co.uk)
Date: Tue Jan 22 2008 - 11:16:05 CST

Dear Thomas,

> due to the larger flexibility of the gaussian input format, I
> would like
> to substitute the divcon program by g03 for the calculation
> of AM1-BCC
> charges used as GAFF parameters of small molecules. In
> prinziple, this
> works pretty well after some modifications of the antechamber
> program.
> But the AM1-BCC charges are much different to the original
> results using
> divcon. Here are the results for ethanol as an example:

As a first guess this may be due the fact that Gaussian uses different
values for a number of constants. Take a look at the supplemental material
of the following paper:

http://www3.interscience.wiley.com/cgi-bin/abstract/117859892/ABSTRACT

Try incorporating the changes described there into G03, recompile it and
then run it again and see if the resulting charges change by an appropriate
amount. They won't come out the same as Divcon since that uses yet another
set of values for the constants so you'd have to make the equivalent changes
in Divcon as well. However, you should be able to gauge how much of an
effect small changes in the values of the constants have on the predicted
charges and so determine if this may indeed be the cause or there is
something else going on.

I note that the two outputs you showed had identical coordinates in both
Divcon and G03. Did you optimize them independently in both to get this? If
you did I find that amazing. However, I assume more likely what you did is
minimize in one and then copy the coordinates over to the other yes? If this
is the case you may want to take that set of "truncated" coordinate (since
they are only to 4 decimal places) and run it through both divcon and
gaussian as a single point calculation to see how much that changes the
charges. It is possible that part of what you see comes from the truncation,
I.e. it was truncated in g03 but Divcon internally was using the full
precision. Again, just a guess.

All the best
Ross

/\
\/
|\oss Walker

| Assistant Research Professor |
| San Diego Supercomputer Center |
| Tel: +1 858 822 0854 | EMail:- ross_at_rosswalker.co.uk |
| http://www.rosswalker.co.uk | PGP Key available on request |

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