Subject: Frozen core definition in Gaussian

From: Kirk Peterson <[email-masked]>
Date: Mon, 7 Jan 2002 16:24:34 -0800 (PST)

All:

after stumbling across something today, I thought that it would
be of enough interest to bring it to people's attention.  Perhaps
this is common knowledge, but I'm guessing it's not.  When
carrying out ab initio calculations on molecules containing 3rd row 
main group elements, i.e., Ga - Kr, the natural definition of the
core electrons for these 3rd row elements (at least in my opinion) 
includes the following electrons: 1s, 2s, 2p, 3s, 3p, and 3d.  (Of course, 
when doing calculations on Ga- or Ge-containing species, one should at 
some point be interested in 3d correlation.)  Much to my surprise,
it appears that the default frozen core in Gaussian does NOT
include the 3d electrons (i.e., the 3d electrons are included in
the valence shell with the 4s and 4p electrons).  Perhaps this was 
to make the definition consistent with the 3d transition metals where 
they are obviously valence, but I would be willing to guess that not 
too many people would guess this choice for an element such as Br.  

Besides probably doing much more work than desired (correlating 10's 
of extra electrons usually adds nontrivial amounts of computational 
expense), most basis sets for these elements do not describe 3d correlation.  
In particular, the cc-pVnZ basis sets for these elements were developed for 
valence correlation only, the definition of which did NOT include the 3d 
electrons of these elements.  So in addition to the added computational expense, 
you might actually do some harm since one may be introducing extra BSSE 
> from correlating core-like electrons with a valence basis set (never a good idea).

Of course, one can change the definition of the frozen core in Gaussian
by using the old ReadWindow (RW) option, but the quick FC (i.e.,
MP2=FC or just MP2) will result in the situation described above.  I guess
one should note that just mentioning in a publication that the frozen core
approximation was used is surprisingly ambiguous...

best wishes,

Kirk Peterson
Washington State University
EMSL/Pacific Northwest Nat'l Laboratory

---

From: Vitaly Rassolov <[email-masked]>
Date: Mon, 7 Jan 2002 21:41:41 -0500 (EST)

A related issue is the definition of core orbitals in molecules such 
as GaF (where 3d "core" orbitals of Ga are energetically above the valence 
2p orbitals of F -> source of many errors).  This and the inclusion of Ga 
3d orbitals into valence space are discussed in two recent articles.  One 
is Published in Chem. Phys. Lett. last month (vol. 350, 
573--576) by Rassolov, Pople, Redfern, and Curtiss; the other one is 
appearing presently in Theor. Chem. Accounts by G. Petersson and 
coworkers. The original problem that prompted the inclusion of Ga 3d into 
valence space was discovered by Radom et al. and Bauschlicher et al. a 
while ago. It looks like we will be able to return 3d orbitals of Ga back 
to its core, without much loss of accuracy.

On Mon, 7 Jan 2002, Kirk Peterson wrote:

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-- 
Vitaly Rassolov
Dept. of Chemistry and Biochemistry         (803) 777-7811
University of South Carolina            fax (803) 777-9521
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