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

A stationary condition involving the first-order wavefunction of many-body perturbation theory (PT) is shown to lead to the partitioning introduced recently by Knowles (J. Chem. Phys., 156, 011101 (2022)). 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 M\o ller-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.