Electron correlation in Li+, He, H− and the critical nuclear charge system ZC: energies, densities and Coulomb holes

This paper presents high accuracy correlation energies, intracule densities and Coulomb hole(s) for the lithium cation, helium, hydride ion and the system with the critical nuclear charge, Z_C, for binding two electrons. The fully-correlated (FC) wavefunction and the Hartree-Fock (HF) wavefunction are both determined using a Laguerre-based wavefunction. It is found that for the lithium cation and the helium atom a secondary Coulomb hole is present, in agreement with a previous literature finding, confirming a counter-intuitive conclusion that electron correlation can act to bring distant electrons closer together. However, no evidence for a tertiary Coulomb hole is found. For the hydride anion and the system just prior to electron detachment only a single Coulomb hole is present and electron correlation decreases the probability of finding the electrons closer together at all radial distances. The emergence of a secondary Coulomb hole is investigated and found to occur between Z = Z = 1.15 and Z = 1.20. The FC and HF energies and intracule densities (in atomic units) used to calculate the correlation energy and Coulomb hole respectively, are accurate to at least the nano-scale for helium and the cation and at least the micro-scale for the anions.