Two new unitary-invariant and size-consistent perturbation theoretical approaches to the electron correlation energy

Using a second quantization representation of the Hamiltonian, an operator representation is derived which corresponds to the indicated partitioning of the CI-matrix in excitation degrees. This gives rise to a new Rayleigh-Schrödinger perturbation theory which is of comparable quality as the well established Møller-Plesset and Coupled-Cluster methods.