A note on the convergence of multiconfigurational many‐body perturbation theory

The convergence of multiconfigurational many‐body perturbation theory ( MC MBPT ) is discussed in connection with the intruder state. Its convergence properties are first examined with a fictitious three‐level system employing a Hermitian version of MC MBPT , which permits a general model space. It is then applied to the H 2 —H 2 and N 2 systems. The results suggest that a more extensive model space is likely to embrace new intruder states and the space extension be executed with due caution.