CCL: negative bsse content

 Sent to CCL by: "Marcin  Makowski" [makowskm-x-chemia.uj.edu.pl]
 Very general thing is that actual LMP2 implementation is an approximation
 to exact MP2 relying (by cut-offs introduced) on localization of orbitals.
 In this method and in hte limit of perfect localization BSSE should just
 disappear. For this reason BSSE here has good chance to be quite small.
 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.
 The result is not physical, but I wouldn't worry too much about it if the values
 are under 1 kcal. If you want to
 get some more confidence, try decreasing cut-off tresholds of the LMP2
 method and see if you obtain smooth behaviour.
 Yours,
 Marcin
 > Sent to CCL by: Jozsef Csontos [jozsefcsontos[a]creighton.edu]
 > Dear List Members,
 >
 > I was calculating intermolecular interaction energies using the
 > counterpoise method to correct the results for bsse (3 dimer centered
 > basis set calcs). The bsse contents were also calculated (2 more monomer
 > centered basis set calcs). If I'm not mistaken I can get the bsse
 > content if I subtract the non-bsse corrected energy from the bsse
 > corrected one. This imply that the bsse content should be positive.
 >
 > I am using quite large basis set considering the investigated systems
 > (~50 atoms, cc-pVTZ, aug-cc-pVTZ) and the BSSE content is usually small,
 > less than 1kcal/mol; the interaction energies are in the 2-8 kcal/mol
 > range. The level of theory are lmp2 and dft.
 >
 > However, in some cases (mainly lmp2 calcs) I got small negative (~
 > -0.3,-0.5 kcal/mol) bsse contents.
 >
 > Do you have any idea, how the bsse content can come to be negative?
 >
 > Thanks,
 > Jozsef
 >
 >
 > --
 > Jozsef Csontos, Ph.D.
 >
 > Department of Biomedical Sciences
 > Creighton University,
 > Omaha, NE