Möller-plesset perturbation theory for excited states of molecules

We consider the criteria that should be met by the perturbation theory for excited states to preserve the advantages of the Möller-Plesset version for the ground state. A few zero-approximation Hamiltonians are proposed and discussed. Expressions for the first-order corrections to the wave function and for the second-order corrections to the energy of excited states are obtained. The expressions are consistent with the conditions of orthogonality of states with the same symmetry. It is shown that the perturbation theory for excited states proposed in this work is identical to the Möller-Plesset perturbation theory for the ground state.