where the operator P2n is a permutation operator that interchanges exactly n pairs of electrons between interacting monomers. The explicit action of the P2n operator for n ≤ min(N A , N B ) on the wavefunction of a dimer leads to the sum of functions with permuted electronic coordinates, each accompanied by the factor consisting of 2n intermolecular overlap integrals S κ µ = φ A κ |φ B µ , where φ A κ and φ B µ represent molecular spinorbitals of monomers A and B respectively. For the case of n ≥ min(N A , N B ), the result of such an action is obviously zero.