2ND-ORDER MOLLER-PLESSET PERTURBATION-THEORY WITHOUT BASIS-SET SUPERPOSITION ERROR

A second order Moller-Plesset perturbation theory which is free of the basis set superposition error (BSSE) is developed based on the ''Chem ical Hamiltonian Approach'' (CHA). The zeroth order Hamiltonian is bui lt up on the BSSE-free (but not orthogonal and not necessarily real) c anonic CHA-SCF orbitals and their orbital energies. As the exclusion o f BSSE makes the problem nonHermitian, biorthogonal perturbation theor y is used to obtain the first order wave function. The second order en ergy is, however, calculated by using the conventional Hermitian Hamil tonian, in accord with the ''CHA with conventional energy'' recipe. Fo r that reason we use a generalized Hylleraas functional introduced rec ently; this guarantees the second order energy to be real even in the case of complex CHA-SCF orbitals. The matrix elements entering the gen eralized Hylleraas functional are calculated by transforming all wave functions, creation and annihilation operators to an auxiliary orthono rmalized basis. The new CHA-MP2 method has been tested on a number of van der Waals complexes and hydrogen bonded systems, by using a variet y of different basis sets. In all cases a remarkable agreement has bee n found with the results given by the Boys and Bernardi's counterpoise method (CP)I this agreement is especially striking in the case of lar ge and well-balanced basis sets. This indicates that the conceptually different CHA and CP schemes both take into account correctly the majo r BSSE effects. (C) 1998 American Institute of Physics.