Perturbative solution of equations of motion for excitation and ionization processes

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Abstract
The equations of motion (EOM) which correspond to electronic excitation or ionization events are analyzed within the framework of perturbation theory. The choice of the Hartree–Fock Hamiltonian as the zeroth order Hamiltonian permits the perturbation equations to be solved in a convenient closed form. A comparison of the results for excitation processes with those given by the random phase approximation (RPA) is made. The role of two particle–two hole excitation operators, which are absent in the RPA, is discussed. Finally, connections are made between the Green’s function approach to calculating ionization energies and the perturbation theory treatment of the EOM corresponding to ionization processes.