Calculation of the energies of interatomic and intermolecular van der Waals interactions in terms of variational perturbation theory

A new quantum mechanical method is suggested for calculating separate contributions to the energies of interatomic and intermolecular van der Waals interactions. The method involves the multipole expansion of the electrostatic potential of two interacting charge systems which is treated as a perturbation. Equations for the induction and dispersion contributions are obtained by varying the second-order correction to the interaction energy. The wave functions of isolated systems in the ground state are taken in single determinantal form. The excited-state wave functions are expressed as superpositions of singly excited configurations (the Tamm-Dankov approximation). The density matrix formalism allowed us to obtain the equations in compact matrix form. The method is applied to calculations of the dispersion energies of interactions between two- and four-electron atoms and between two benzene molecules.