The central endeavour of electronic structure theory is the efficient and accurate calculation of the quantum mechanical behaviour of correlated many-body systems, beginning with the ground state In wavefunction theoretical (WFT) approaches, this is traditionally done by first considering a mean-field reference -usually Hartree-Fock (HF) and then recovering the missing correlation energy from its one-electron components. Beyond this common beginning, the realm of post-HF WFT methods is vast, with the most prominent members being Coupled-Cluster (CC) theory, 2,5-10 configuration interaction (CI) methods and many-body perturbation theory (MBPT). While high levels of CC theory are commonly employed for benchmark calculations, they can quickly become prohibitively expensive for large systems. CI, on the other hand, is often useful as a model expansion and as a starting point for multireference approaches (e.g. CASCI and MCSCF) , but its original formulation has fallen out of favour as it is not size-consistent or size-extensive when truncated by level of substitution. While there are many variants of MBPT, most modern perturbation theories distinguish themselves by specific definitions of the physics of the hypothetical unperturbed system, but follow a standard Rayleigh-Schrödinger (RS) approach for the derivation of energy and wavefunction correction terms. The common example of this class is Møller-Plesset perturbation theory. At second order, MP2 is computationally economical, as a simple implementation scales as O(M 5 ) with molecular size M as well as size-consistent, and size-extensive. However, MP2 diverges for zero-gap systems, tends to overestimate the correlation energy for finite systems and cannot restore spin symmetries if they are broken in the reference. These disadvantages can be partially remedied, for example by orbital optimisation (e. g. OOMP, κ-OOMP , etc.), and/or spin-scaling approaches (e. g. SCS-MP , SOS-MP , etc.), and/or the use of explicit regularisation (e. g. κ-MP , σ -MP , etc.).