eq:gfortho eq:gfortho with d i j Kronecker delta. The product function Y k is fully characterized once the set of states k ⌘ (r, s,...) is made explicit. No inter-molecular correlation is explicitly included in a wave function of the form in eq. ( ), since electrons belonging to different groups are assumed to move independently, but a large part of intra-molecular correlation may be accounted for by admitting a some degree of configuration interaction within each electron group.