Subject: RE: CCL:RELATIVITY IN COMP CHEM

From: "C. Brian Kellogg" <[email-masked]>
Date: Tue, 9 Sep 1997 13:36:56 -0400 (EDT)

Dear Dr. Lewars-

	In reference to your questions about relativity in quantum chemistry

> 1)  Are there cases where the Dirac-Fock equation, rather than the
>     Schroedinger equation, should be used?
>
Any molecular system which has heavy elements or exhibits large spin-orbit
or spin-spin interactions should be more adequately described by Dirac-Fock
based methods.  Because of the significantly larger basis set requirements
of the Dirac-Fock method, however, effective core potential methods are
typically employed to estimate relativistic effects.  

> 2)  To what extent are the effective core potentials of ab initio
>     calculations derived empirically, to fit experiment, rather than
>     theoretically?

Effective core potentials are constructed solely with reference to the
theoretically derived atomic structure.  In brief, the core potentials are
derived such that when they are employed in the atomic case, they provide the
best possible reproduction of the valence atomic wavefunction obtained for
the all-electron case.  

> 3)  Where should you start using effective core potentials to simulate
>     relativistic effects --at which element does the error start becoming
>     significant (somewhere around element 33 to 37, I suspect--As, Se, Br,
>     Kr, Rb)?

Small errors in equilibrium properties can even show up in the third row,
and spin-orbit effects may be found throughout the periodic table.
Generally, ECP based methods are thought to be effective for the estimation
of relativistic effects on most molecular properties for Z = 19-36, and even
up through Z=54, but that for heavier elements, the agreement between
Dirac-Fock predictions and ECP predictions is often poor.

If you would like to see a more lengthy discussion of these topics, I have
an  on-line introduction to relativistic quantum chemistry which should
provide you with more in-depth answers as well as some references. It can be
found at:

http://zopyros.ccqc.uga.edu/~kellogg/docs/rltvt/rltvt.html

In particular, you may find the final section of that web page, "Relativistic 
Effects in Atoms", more helpful.  That section presents some graphs of the 
magnitude of relativistic effects for main group elements and discusses the 
major trends of relativity across the periodic table.

I hope this helps.

-Brian Kellogg

-- 
C. Brian Kellogg          Physics Building (221), office A367
phone: (301)975-2526      National Institute of Standards and Technology
fax:   (301)975-3670      Gaithersburg, MD 20899