Subject: Electrostatic potential questions

From: Luis Montero <[email-masked]>
Date: Tue, 4 May 1993 5:28:05 UTC+0100

Dear netters:

The interesting finding of Arteca's paper is, from my point of view a casual
quality of MNDO. Let us take into account that this method took a consistent
approximate hamiltonian (Pople's NDDO, 1965-67) and parametrized it to give
fair results for a fixed set of molecules which experimental geometries and
heats of atomization were accurately determined. This work have been done
for each element. It gave no meaning to different terms in hamiltonians, if
you take them separately, but a certainly confident result for tarjet proper-
ties (geometries and heats of formation, known the experimental heats of
formation of gaseous elements) of the molecules. Electrostatic fields depend
on molecular eigenfunctions, and they could be good from these grounds.

AM1 and PM3 (truly AM1, beacuse PM3 is a reparameterization) introduced some
terms in the distance matrix dependent term after the SCF procedure. It was,
in the pure molecular SCF, the core-core interaction term of the Born Oppen-
heimer approximation. Many theoreticians of semiempirical methods attribute
to this term some of the frequent wrong results. In fact, this distance matrix
dependent term is theoreticaly consistent, but it is used for introducing
corrections to the approximate SCF results when geometrical quantities are
calculated. AM1 used a variable set of "forza bruta" (Dewar's words) gaussian
terms in this part of the calculation to improve results for certain non pure-
ly bonding cases as hydrogen bridges and cyclic CH structures. It, consequen-
tly, afected parameters in the SCF hamiltonian. Probably, these extrange
inclusion in the full SCF procedure affected more the quality of molecular
eigenfunctions than the former parameterization of MNDO.

I have some non published results where it is clearly seen the lowest quality
of molecular wave functions by PM3, being molecular geometries OK.

Best regards,

Luis Montero
Universidad de La Habana ([email-masked])
CSIC, Madrid ([email-masked])