1 since the resolution of identity for one electron wavefunctions appears in these expressions. In this form it is possible to recognise that the first term in equation ( ) represents actually a true form of P4 operator, lacking only the correct prefactor. The proper coefficient can be easily established, since a P2n is a n-electron operator with respect to the monomer A and likewise for monomer B, two 1/(n!) factors should appear. Additionally a correct sing is given by the parity of the number of permuted electron pairs. Hence the prefactor for the P2n operator is