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Date: Tue, 20 Mar 2007 10:00:31 -0700 (PDT)
From: Francesco Pietra <chiendarret@yahoo.com>
Subject: [NWCHEM] Partial atomic charges
To: users nwchem <nwchem-users@emsl.pnl.gov>
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I am setting up classical (Amber FF) simulations on
natural products, where the "fragment" is the final
"sequence" approaching the limit of 99 atoms.
Difficult, if possible at all, to cut the molecule
into fragments.

In learning about the method used to define the
partial charges, I came across a discouraging (for my
type of job) description:

"One of the weak points of CHELPG (and other
approaches based on fitting the MEP) is the treatment
of larger systems, in which some of the innermost
atoms are located far away from the points at which
the MEP is evaluated. In such a situation, variations
of the innermost atomic charges will not lead to
significant changes of the MEP potential outside of
the molecule and fitting of these atomic charges will
therefore not result in meaningful results. It should
also be remembered that atomic charges can depend
strongly on the conformation of the molecule.
Representative atomic charges for flexible molecules
should therefore be derived as average values over
several conformers. Using ethanol as an example, the
charge parameters derived for the Cs symmetric
conformer A and the C1 symmetric conformer B vary, in
part, by almost 30%. The variation is, however, rather
different for different parts of the
system(http://www.cup.uni-muenchen.de/oc/zipse/compchem/pop/chelpg.html)"

At any event, for a given novel fragment (geometry
optimized with MM global space search), should the
RHF-ESP calculation be preceded by a RHF-Geometry
calculation?

I rely on Ecce for the ESP calculation, though it
seems to me that the question I posed is valid also
for using NWChem directly.

Thanks
francesco pietra






 
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