Subject: Effective Core Potentials

From: Dale Braden <[email-masked]>
Date: Mon, 9 Sep 1996 12:56:40 -0700 (PDT)

Dear CCL,

I read the postings of a year or two ago regarding the use of effective 
core potentials in ab initio calculations, and I would like to submit a 
few questions on this subject.

As I understand it, ECPs were developed primarily to speed up
calculations by reducing the number of electrons that needed to be
considered independently.  An added benefit was that relativistic energy
corrections could then be made for the electrons in the core.  Since core
electrons aren't thought to participate much in bonding, this was
justified.  So my first question is, If I am doing a *geometry
optimization* (and don't care about what the energy is) on an
organometallic complex, and can afford to use an all-electron basis,
should I do so?  In other words, does the use of an ECP generally have a 
neglible effect upon the geometry? 

I know that relativistic effects increase with atomic number.  Are such 
effects already drastic on first-row transition metals, or do they become 
significant (in terms of energy or geometry) only by the second-row?

After looking over the literature on this subject, I am confused as to
whether there is a difference between an "effective core potential" and a
"pseudopotential" for the core.  I use Gaussian 94/Rev. C.3, and when I
tried to input the PPs of Dolg, et al. [JCP 86 (1987) 866], the program
would not accept them, but seemed to be expecting information on the
different "projections", like P-D, S-D, and so forth.  I'm not sure what
these are, and in any case, they are not explicity described by Dolg, et
al., so I don't know whether the problem is one of format or not.

I shall collect and post all responses!

Thank you,

Dale Braden
Dept. of Chemistry
University of Oregon
[email-masked]