pol represents a correction of n-th order in the intermolecular interaction operator V . Such an approach is usually referred to as the polarisation expansion or polarisation theory. Since the reference wavefunction ψ (0) , due to its product form, is not antisymmetric with respect to permutations of all electronic coordinates, a procedure of forcing correct permutational symmetry of the wavefunction was essential to develop (see Refs. 1, 10, and 11).