Applicability of multi‐reference many‐body perturbation theory to the determination of potential energy surfaces: A model study

The recently written CI ‐based multi‐reference many‐body perturbation theory ( MR‐MBPT ) program package is exploited to study a simple ab initio minimum basis set model involving four hydrogen atoms in a rectangular configuration. This model was examined earlier by several authors using both coupled cluster ( CC ) and finite‐order MBPT approaches. Here we present the MR‐MBPT results up to the 50th order and examine the effect of various shifting techniques on the convergence behavior of this approach. It is shown that in contrast with CC methods, both single and MR finite‐order MBPT potential energy calculations are plagued with convergency and intruder state problems, which can be particularly severe when the latter approach is employed for non‐degenerate situations.