This not only includes various flavours of DFT, including the original local-density approximation functionals, the nowadays ubiquitous hybrid functionals such as B3LYP and PBE0, and the modern double-hybrid functionals of Truhlar, Grimme and others, but also post Hartree-Fock (HF) methods, such as Møller-Plesset perturbation theory and coupled cluster theory. In the latter case, the popularity of these post-HF wavefunction based methods has been greatly accelerated by time-saving approximations, such as the resolution of the identity (RI) approximation, and the closely related chain of sphere exchange algorithm (COSX), permitting their application to larger systems than has been possible previously. These developments, amongst others, have led to computational chemistry being routinely used to understand natural phenomena that are otherwise inscrutable, and to predict the properties of molecules, either as a screening tool to help guide synthetic efforts, or to provide additional evidence for synthetic pathways or molecular properties.