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-mu v, density cumulants lambda(mu v rho sigma) and two-electron integrals [mu v vertical bar rho sigma] 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. (c) 2006 Elsevier B.V. All rights reserved.