Subject: BBSE corrections

From: [email-masked]
Date: Tue, 30 Jan 2001 11:51:45 -0400 (EDT)

Hello CCLers,

Are there circumstances under which it is possible to have an increase in
energy after employing a counterpoise BSSE correction?  Over the course
of my calculations, I am consistently obtaining higher energies after
corrections are applied.  The system I am studying includes an ECP (SDD
large core) and a Pople basis set (6-31+G(2d,p).  Is it possible that the
use of a mixed basis set may affect the correction?  I have not seen this
anywhere and, instinctively, I would assume that as basis sets are added
to a system, the resultant energy should be lowered.  

Thank you for your time.

Sean Hughes
Centre for Research in Molecular Modeling (CERMM)
Dept. Chemistry and Biochemistry
Concordia University
1455 de Maisonneuve Blvd. West
Montreal, Quebec
Canada
H3G 1M8
Tel: (514)-848-4260
Fax: (514)-848-2868

---

From: TREVOR D POWER <[email-masked]>
Date: Tue, 30 Jan 2001 12:37:12 -0600 (CST)

Sean,
	To respond to your first question: It really depends where you are
on the potential energy surface when you apply the conterpoise correction
method.  Note that the minimum obtained from a geometry optimization
(where BSSE correction is necessary) results from the unphysical
interaction between orbitals (occupied and unoccupied) of monomer A and B.
This unphysical interaction of orbitals places the optimized geometry such
that monomers A and B are closer than they should be.  If this equilibrium
point is where you apply the counterpoise correction method, you will find
that the BSSE corrected energies are higher than they were before the
correction was applied.  

Hope this helps,
David Power
Department of Chemistry
University of North Texas
NT Station, Box 305070
Denton, Texas 76203-5070
[email-masked]

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