The Ring and Exchange-Ring Approximations Based on Kohn–Sham Reference States

Abstract

The ring or random-phase approximation (RPA) method combined with Kohn–Sham reference states has become established as an alternative method to common ab initio wave function methods for the description of the electronic structure of molecules and solids. The reason for this lies in the fact that the RPA possesses, in contrast to, for example, configuration interaction or coupled-cluster methods, a favourable scaling behaviour of N ⁴ ⋅ log (N) with the system size and describes a number of thermodynamic and electronic properties with a higher accuracy than standard density-functional theory methods. Moreover, the RPA method is able to describe not only dynamic but also strong static electron correlation effects, in contrast to conventional single-reference methods. The latter also include large systems with a small or vanishing band gap. In this work, the performance of the RPA and some extensions to the RPA, including exchange correlations, are tested for the description of thermochemical properties.