Subject: Ab Initio Theory of Semiempirical Methods

From: [email-masked] (Charles Martin)
Date: Tue, 19 Apr 1994 13:55:56 -0600 (CDT)

Netters,

        More on the discussion about semiempirical methods:

        I have been working with Karl Freed at the University of Chicago
also trying to develop a "new model" for semiempirical methods.  We have
had quite a bit of success producing semiempirical parameters for
pi-electron systems soley from correlated ab initio calculations
using the effective valence shell Hamiltonian (Hv) method.  Note
that our ``parameters'' involve no tweaking and no number-twiddling
(Some of this will come out in JCP soon, but right now the
papers are tied up in press. They will also be available
online via JCP express.  Of course, we will be happy to send anyone
reprints or preprints if they are interested)  The Hv method provides
a rigorous derivation of the semiempirical Hamiltonians directly
>from the Schr\"odinger Equation.  Granted, we are not
not upto the all-valence-electron case, but there are two reasons for that.
First, the computations are very complicated and slow, and second, the
standard semiempirical Hamiltonians seem to average three-electron
correlation contributions which appear in the exact theory.  We do not
yet understand the all of the details of this averaging, and as a result,
we can not yet produce all-valence electron parameters.  We are very close
to solving this problem, but it is not trivial.  From our
point of view, there is alot more going on in these methods than just
number-twiddling, and there is a lot of room for advancement in both the
ab initio theory of semiempirical methods and the computational implementation
of the effective Hamiltonian method.

Also, from our studies of semiempirical theory, we have learned how to
perform better ab initio calculations. The Hv method is a decent
multi-reference ab initio method, albiet it is a bit slow in it's current
implementation.

We would be happy to discuss some of these problems with researchers
trying to build new semiempirical methods, and we also have a post doc
position (see the CCL job listings) open in this area.
I would also like to know if this work is being duplicated. I know
that some people use effective Hamiltonians for Ligand Field Theory,
Effective-Spin Hamiltonians, etc.  

==================================================================
Charles H. Martin

email: [email-masked]

US Mail:
        c/o Freed Group
        The James Franck Institute and 
        The Department of Chemistry
        The University of Chicago
        5640 South Ellis Avenue
        Chicago, Illinois  60637

Work:  (312) 702-3457
Fax:   (312) 702-5863  
==================================================================