Ground state of two electrons on concentric spheres

We extend our analysis of two electrons on a sphere to electrons on concentric spheres with different radii. The strengths and weaknesses of several electronic structure models are analyzed, ranging from the mean-field approximation (restricted and unrestricted Hartree-Fock solutions) to configuration interaction expansion, leading to near-exact wave functions and energies. The Møller-Plesset energy corrections (up to third-order) and the asymptotic expansion for the large-sphere regime are also considered. We also study the position intracules derived from approximate and exact wave functions. We find evidence for the existence of a long-range Coulomb hole in the large-sphere regime and infer that unrestricted Hartree-Fock theory overlocalizes the electrons.