Fast approximation of the first-order intermolecular interaction energy:: ab initio calculations using biorthogonal orbitals

The first-order intermolecular interaction energy is computed from a second-quantization-based perturbation method using biorthogonal orbitals. This symmetric treatment allows fast ab initio, calculations of the first-order intermolecular interaction energy. In the present stage of the work, the effects of the intramolecular correlation are neglected. The truncation of the basis set (leading to the well-known BSSE in supermolecule treatments) is corrected by using the basis set of the dimer for the monomers. The calculation of the long-range electrostatic term also allows to define an exchange term corresponding to the use of a symmetry adapted perturbation method. The meaning of the electrostatic and exchange terms in these two kinds of decomposition is discussed. The relation with semi-empirical approximations is commented. (C) 2001 Elsevier Science B.V. Ail rights reserved.