Hi Olegas,
 > In the "standard" BSSE equation (for a dimer AB):
 > BSSE(AB) =3D E(AB)^AB - E(A)^A - E(B)^B + [E(A')^A - E(A')^AB + E(B)^B - =
 > E(B)^AB]
 > I've got most of the parameters, but how in G98W do I calculate E(A)^AB =
 > and E(B)^AB, i.e. the energy of the monomer units in terms of the dimer =
 > basis set.
 With Gaussian, the keyword Massage has to be used for counterpoise
 corrections (assuming it's the same for G98 and G98W).  Have a look at the
 Gaussian manual:
 There is an example there as well. However, don't believe the sentence
 about counterpoise being only a crude estimate, have a look at the
 following papers to learn more about BSSE and Counterpoise:
 -  F.B. van Duijneveldt, J.G.C.M. van Duijneveldt-van de Rijdt, and
    J.H. van Lenthe, Chem. Rev. 94, 1873, 1994 (on counterpoise theory)
 -  S. Simon, M. Duran, J.J. Dannenberg, J. Chem. Phys. 105, 1996
    (on geometry optimizations on BSSE-corrected potential energy surfaces).
 -  F.B. van Duijneveldt, "Basis Superposition Error"
    in "Molecular Interactions" S. Scheiner (ed.), Wiley (1997)
 -  T. van Mourik, A.K. Wilson, K.A. Peterson, D.E. Woon, T.H. Dunning, Jr.,
    Adv. Quant. Chem. 31, 105, 1999 (showing BSSE effects on interaction
    energies, distances, and frequencies).
      Tanja van Mourik
      Royal Society University Research Fellow
      Chemistry Department
      University College London    phone:    +44 (0)20-7679-4663
      20 Gordon Street             e-mail:   work: T.vanMourik "at@at"
      London WC1H 0AJ, UK                    home: tanja "at@at"