Ab initio energy partitioning at the correlated level

The Mayer energy partitioning is extended to the case of a general ab initio wavefunction. The key idea is to expand the two-electron energy in terms of the density matrix elements D_μν, density cumulants λ_μνρσ and two-electron integrals [μν{divides}ρσ] and to partition the cumulants into one-atom and two-atom components. The numerical results are shown at the CI-D level and are compared to those of the Hartree-Fock energy partitioning.