Communication: Coupled cluster and many-body perturbation theory for fractional charges and spins

The study of systems with fractional charges and spins has become an extremely important tool to understand errors in approximate electronic structure methods, particularly in the context of density functional theory. Meanwhile, similar studies with wavefunction (WF)-based methods beyond second-order perturbation theory have been lacking. In this contribution, we study the performance of different coupled cluster (CC) and many-body perturbation theory (MBPT)-based methods for fractional charges. The use of the conventional and renormalized formulations of fractional-charge MBPT is discussed. The fractional spin behavior of the coupled cluster doubles (CCD) method is also investigated. Overall, all tested WF methods show very promising performance for the fractional charge problem. CCD is also quite accurate for the fractional spin problem in He⁺ across most of the range, although it breaks down to near Hartree-Fock quality in the strongly correlated limit. Beyond the study of fractional charge and spin curves, the implementation of CC methods with fractional occupation numbers offers a promising route to treating problems with multi-reference character in a single-reference framework.