Knowles Partitioning from a Stationary Condition: Single- and Multireference Case

A stationary condition involving the first-order wave function of many-body perturbation theory (PT) is shown to lead to the partitioning introduced recently by Knowles (<i>J</i>. <i>Chem</i>. <i>Phys</i>., <i>156</i>, 011101 (<b>2022</b>)). This facilitates direct generalization for multireference (MR) PT schemes operating with a one-body Hamiltonian at zero-order. The essence of the method is an optimization of one-body integrals in the first-order interacting subspace, thereby achieving superior performance over Møller-Plesset (MP) type approaches. The stationary condition based extension, performed in the pivot-independent variant of the multiconfiguration PT (frame MCPT, fMCPT), rectifies the shortcomings of our previous MR adaptation. The resulting PT series comes close to the stationary condition-based extension, carried out in the complete active space PT (CASPT) formalism. Numerical results demonstrate that Knowles partitioning consistently outperforms MP partitioning in fMCPT.