Solving the mean field Hartree-Fock (HF) equation self consistently gives the lowest energy single Slater determinant electronic wave function. To solve the many-body problem, one needs to include the missing correlation energy. Second order Møller-Plesset (MP2) perturbation theory 66,67 offers a useful and computationally inexpensive approximation to treat the correlation yielding the following expression in the case of restricted HF orbitals: