Equations ( ) and ( ) below present the resulting working formulas expressed in terms of molecular orbitals. These expressions assume real orbitals and the closed-shell restricted Hartree-Fock (RHF) (or restricted Kohn-Sham (RKS)) reference. The labels a, (b) refer to occupied orbitals, while r, (s) to virtual orbitals of monomer A, (B). Summation over the repeated indices is implied in all expressions. The first-order polarization correction to the IIP reads