Subject: Results (Was: Gaussian "Alter" Keyword)

From: "Phil Hultin" <[email-masked]>
Date: Wed, 19 Feb 2003 09:23:29 -0600

I was interested by the recent question about performing a calculation on a
high-spin Fe(II) complex.  I don't have any "technical" advice to offer as
to how to get the calculation to work - that has already appeared on CCL.
What caught my eye was the following:
  "Unfortunately the results I obtain even with the small system are hardly
believable. The frontier orbitals do not resemble d orbitals very much and
the spin density is not concentrated on iron (only 1/4 of it is on Fe)."

The quality of the calculation may well be suspect, owing to the problems in
converging, but is it really appropriate to judge the performance of a
computation by how closely it reproduces the qualitative pictures we use in
introducing undergraduates to electronic structure theory?  In most cases
that I have examined, the HOMO and LUMO do not resemble hydrogen-like atomic
orbitals, nor would I expect them to.  These are, after all, MOLECULAR
orbital calculations.  The symmetry properties of these frontier orbitals
obey certain rules, but I don't know of any reason to expect them to look
very much like traditional pictures of atomic orbitals.

I also wonder if the fact that spin density given by the reported
calculation is not concentrated where we would draw it on paper is all that
significant.  After all, although in organic chemistry we tend to draw
methyl cation (CH3+) with the charge on carbon, calculations and other data
suggest that in fact the bulk of the charge is on the hydrogens.

Most experimentalists use models that are only rough approximations to
reality - I include myself in that group, since I am primarily a synthetic
organic chemist.  But, isn't at least part of the point of doing ab initio
calculations to find out more about what may be going on "under the hood" in
chemical structures?

What do people think about the relationship between simple qualitative
models, more sophisticated ab initio models, and "reality" (whatever that
may be)?

Dr. Philip G. Hultin
Associate Department Head and
Associate Professor of Chemistry
University of Manitoba
Winnipeg, MB, Canada R3T 2N2
(vox) 204-474-9814
(fax) 204-474-7608
mailto:[email-masked]
http://www.umanitoba.ca/chemistry/