Knowles Partitioning at the Multireference Level.

A multireference generalization of the partitioning introduced recently by Knowles ( 2022 156, 011101) is presented with the aim to study electronic systems at a medium level of correlation. The multireference formalism applied is the general framework of multiconfiguration perturbation theory (MCPT).

Analysis and Assessment of Knowles' Partitioning in Many-Body Perturbation Theory.

A detailed analysis of a new partitioning in many-body perturbation theory recently proposed by Knowles (, 011101, ), termed "perturbation adapted partitioning" (PAPT), is presented. Level shift and orbital rotation effects are identified as gears of the zero-order Hamiltonian. These two components are examined separately, revealing that, in themselves, neither of the two is competitive with the combined effect.

Pivot invariance of multiconfiguration perturbation theory via frame vectors.

Multiconfiguration perturbation theory (MCPT) is a general framework for correcting the reference function of arbitrary structures. The variants of MCPT introduced so far differ in the specification of their zero-order Hamiltonian, i.e.

Many-Body Perturbation Theory with Localized Orbitals: Accounting for Localization Diagrams as Integral Dressing.

We argue that the so-called localization diagrams, originating from off-diagonal Fockian elements, do not have to be dealt with explicitly in the Davidson-Kapuy many-body perturbation theory with localized orbitals but can be accounted for by dressed two-electron integrals.

A note on perturbation-adapted perturbation theory.

The partitioning introduced recently by Knowles [J. Chem. Phys.