Subject: Re: CCL:RELATIVITY IN COMP CHEM

From: [email-masked] (Tom Cundari)
Date: Tue, 09 Sep 1997 12:10:22 -0600

Hello,

Based on my experience with ECPs I can comment, most directly on questions
2) & 3)

2) ECPs are generally derived to model the potential generated by core
electrons in an atom.  If one starts with a Dirac-Fock calculation then the
ECP models a 'relativistic' field. Likewise, if one starts with a
Hartree-Fock calculation then a 'nonrelativistic' field is modeled by the
ECP.  In the ECPs developed by Stevens et al. all elements larger than Ne
are generated from Dirac-Fock calculations.  The use of relativistic
calculations to generate the ECP is employed by Hay-Wadt (Rb and higher)
and Ross et al. (Li and beyond).  To my knowledge, most ECPs are derived
empirically from atomic calculations and not adjusted to fit experimental
data.  I am not sure about the so-called "energy-adjusted ab initio
pseudopotentials" from the Stuttgart group, but their works can be
consulted for specifics.

3) Depends, of course, on what degree of accuracy you are looking for.  For
'chemical'
accuracy (prediction of geometries and some energetic quantities), my
experience suggests that you probably do not have to worry about
relativistic effects until the third transition series.  For highly
accurate 'spectroscopic' quantities (e.g., state-to-state transition
energies) your surmise about the 4th period elements is probably correct.

In my opinion, the best work on the use of ECPs to probe the importance of
relativistic effects
has been done by Schwerdtfeger & by the Stuttgart group.  In their work,
they cleverly use
a combination of 'relativistic' & 'nonrelativistic' ECPs to probe the
importance of
relativistic effects.    Some papers by Schwerdtfeger et al. are

38.     a)J. Am. Chem. Soc., 111, 15 (1989).
        b)  J. Am. Chem. Soc., 114, 7518 (1992).
        c) Organometallics, 9, 504 (1990).

In the following paper by Dolg et al. they study CeO.  An increase in bond
length is seen upon inclusion of relativistic effects as opposed to the
more common relativistic bond contraction.

J. Chem. Phys., 90, 1730 (1989).

For anyone interested in relativistic effects in chemistry, the reviews by
Pyykko are classic.  I think they remain the best intro to the topic for
those with a 'chemical' interest in relativity.

a) P. Pyykkö, Chem. Rev., 88, 563 (1988).  Relativistic Effects in
Structural Chemistry.
b) P. Pyykkö, Adv. Quantum Chem., 11, 353 (1978).  Relativistic Quantum
Chemistry.

>1997  Sept 9
>
>Hello,
>
>Concerning the importance of the theory of relativity in computational
>chemistry:
>
>1)  Are there cases where the Dirac-Fock equation, rather than the Schroedinger
>    equation, should be used?
>2)  To what extent are the effective core potentials of ab initio
>    calculations derived empirically, to fit experiment, rather than
>    theoretically?
>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)?
>
>    Thanks
>
>           E. Lewars

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Tom Cundari                                           Department of Chemistry
Associate Professor                                The University of Memphis
e-mail:[email-masked]           Memphis, TN 38152-6060
phone: 901-678-2629
FAX: 901-678-3447
http://www.chem.memphis.edu/umchem.html

****  U of Memphis is conducting a search for an Assistant Professor
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