A comment on a theory of electron affinities

Simons and Smith have calculated electron affinities and ionization potentials through third order in the electronic interaction by invoking the equations of motion method. An alternative analysis of the order is offered here by expanding the denominator in their pseudoeigenvalue equation to generate a Rayleigh-Schrödinger-like perturbation expansion. It is shown that considering the off-diagonal terms in the self-energy denominator leads to terms not considered by Simons and Smith which are third order in the electronic interaction as counted in our expansion. The connection with the diagram methods of Cederbaum is made.