Subject: summary of semi-empirical calculations post....

From: Stevie T <[email-masked]>
Date: Tue, 19 May 1998 11:12:15 +0100

Cheers for the replies regarding my question on dipole moments. The
replies are all summarised below.

My original post :

Hi all...

I have recently been doing some dipole moment calculations using the AM1

and PM3 semi-empirical methods in Hyperchem. I have been trying the
caculations at different torsion angles and obtaining some reasonable
results. However, occasionally a difference as small as 10 degrees makes

the dipole moment value increase from 4D to about 4000D. Has anyone else

had this problem ? What do people think of the accuracy of
semi-empirical calculations for such properties as dipole moments ?

Also, when the calculations were repeated under Mopac/AM1 and Mopac/PM3
in Cerius2 they yielded reasonable results. I presume this is to do with

the individual ways in which the two similar methods are implemented
under different packages (ie Hyperchem and Mopac). Has anyone any ideas
on this or any references to work done on this ?

==================

I'm not an expert in computational chemistry, but you might want to look

at
an article by Storer et. al. J. Comp.Aided Mol. Design 9 (1995) 87-110
where
roughly 200 AM1 and PM3 dipole moments for different neutral molecules
are
compared with those obtained from an alternative charge mapping method.
The
RMS errors reported in this article are 0.30 D and 0.26D respectively
for
the AM1 and PM3 semi-empirical methods. The point of the article is to
offer
an alternative charge model with improved accuracy.  Hope this helps
answer
your question.

Brian Williams, Chemistry, Bucknell University

======================
>From : Dr. Jose Luis Garcia de Paz

Hi

Mulliken charges an dipole moments when heteroatoms are
included are often unreliable at PM3 level due the parametrization.
They are much better at AM1 level . However PM3 geometries are
usually better than the AM1 ones. Regards

Jose

==================

This seems to be a bug in Hyperchem. Semiempirical dipole moments may
be not perfect, but they are not *that* bad. In fact, 4000D molecular
dipole moment would be very strange coming from any method: it implies
that your molecule has two electron-sized local charges separated by
almost 1000 angstrom. Or two 10e charges at 100 angstrom, etc.

Regards,

/Serge.P
===================
--

"I used to think the brain was the most important organ in the body,
'til I realized yeah, look what's telling me that."
                               -- Emo Phillips

        mail to : [email-masked]
        web-site: http://www.hull.ac.uk/php/chpsmt/

--

"I used to think the brain was the most important organ in the body,
'til I realized yeah, look what's telling me that."
                               -- Emo Phillips

        mail to : [email-masked]
        web-site: http://www.hull.ac.uk/php/chpsmt/