# CCL: negative bsse content

*From*: Jozsef Csontos <jozsefcsontos-x-creighton.edu>
*Subject*: CCL: negative bsse content
*Date*: Wed, 31 May 2006 13:51:39 -0500

Sent to CCL by: Jozsef Csontos [jozsefcsontos%%creighton.edu]
Hi Tanja,
> Dear Joszef,
>
> BSSE is a negative quantity: it is calculated as the difference between
> the sum of the monomer energies calculated in the dimer basis set and
> the sum of the monomer energies calculated in the monomer basis set, all
> of these at the geometries they adopt in the complex.
> Perhaps some formulas can help
> (with latex-style sub- and superscripts):
>
> BSSE = E_{A}^{AB}(AB) + E_{B}^{AB}(AB) - E_{A}^{A}(AB) - E_{B}^{B}(AB)
you are right, equations can help, sorry for neglecting them.
I calculated the interaction energy according to the original
Boys-Bernardi procedure:
DeltaE(CP)=E_{AB}^{AB}(AB) - E_{A}^{AB}(AB) - E_{B}^{AB}(AB)
in this case the non corrected interaction energy is
DeltaE(noCP) = E_{AB}^{AB}(AB) - E_{A}^{A}(A) - E_{B}^{B}(B)
The difference between the two is the bsse content, which is the same as
you stated above.
Some words about the sign, I think the sign of the bsse is the matter of
definition:
(+) JCC, 25, 1771, 2004
(-) JCC, 22, 196, 2001
I used the DeltaE(CP)-DeltaE(noCP) equation which implies that the bsse
is positive. This gave me results with opposite sign comparing to yours:
> DeltaE(CP) = DeltaE(noCP) - BSSE
>
As you might noticed, I didn't correct for deformation (fragment
relaxation), but it vanishes anyway when you subtract the interaction
energies.
So, apart from the signs, I should always get results on the same side
of zero, but I didn't.
Best wishes,
Jozsef
PS.: I tend to accept Marcin's opinion.
>I wouldn't be suprised if subtle changes in the shape of localized
>orbitals and effective partition of space between separate computations
>could actually transform some small positive net effect into small
>negative.
--
Jozsef Csontos, Ph.D.
Department of Biomedical Sciences
Creighton University,
Omaha, NE