Communication: Beyond the random phase approximation on the cheap: improved correlation energies with the efficient...

The "ACFD-RPA" correlation energy functional has been widely applied to a variety of systems to successfully predict energy differences, and less successfully predict absolute correlation energies. Here, we present a parameter-free exchange-correlation kernel that systematically improves absolute correlation energies, while maintaining most of the good numerical properties that make the ACFD-RPA numerically tractable. The radial exchange hole kernel is constructed to approximate the true exchange kernel via a carefully weighted, easily computable radial averaging. Correlation energy errors of atoms with 2-18 electrons show a 13-fold improvement over the RPA and a threefold improvement over the related Petersilka, Gossmann, and Gross kernel, for a mean absolute error of 13 mHa or 5%. The average error is small compared to all but the most difficult to evaluate kernels. van der Waals C(6) coefficients are less well predicted, but still show improvements on the RPA, especially for highly polarisable Li and Na.