| From: |
apa -8 at 8- pop.cc.adfa.oz.au |
| Date: |
Tue, 3 Aug 1993 13:39:48 +1000 |
| Subject: |
Re: ESP-fitted Point Charges |
In reply to "M.E.Davis and any other guy in state of confusion"
Modesto Orozco doctor.chem.yale.edu> writes:
>
>A lot of people is using semiempirical methods to obtain MEPs and then fit the
>ESP charges. The semiempirical ESP and ab initio charges correlate well and
>then
>it was suggested to use the slope of the regression line abinitio = c* semiemp
>as a scaling factor. The charges are determined at the smiempirical level
>(keepi
>ng the constrain that SUMQ=0.0) and after this are scaled, since SUMQ=0,
>SUMQsca
>l are 0 and the monopole is conserved (not the dipole of course).
^^^^^^^^^^^^^^
I would like some clarification on this highlighted point. The ESP'ified MOPAC
that I'm using will produce a set of atom-centered point charges which are
least-squares-fitted to reproduce the MOPAC semiempirical electrostatic
potential. If the DIPOLE or DIPX/DIPY/DIPZ keywords are used, the fitting
procedure will also be constrained to reproduce the dipole moment (or x-,
y- or z- components).
MOPAC seems only to allow this constraint if the molecule is neutral.
Why is this?
Taking the ammonium ion as an example, why cannot the semiempirical esp be
fitted to a set of 5 N- and H-centered point-charges, chosen so that the
overall charge is +1 and dipole moment is zero in x-, y- and z- directions?
----
Alan Arnold | e-mail: apa ^at^ pop.cc.adfa.oz.au
Chem. Department,University College | voice : +61 6 268 8080
Australian Defence Force Academy | fax : +61 6 268 8002
CANBERRA ACT 2601 Australia |
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