Second-order electron correlation energies for Zn2+ and Zn

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Abstract
Second‐order Rayleigh–Schrödinger Hartree‐Fock perturbation theory is applied for an extensive study of the correlation energies of Zn2+ and Zn in order to analyze the nature of various correlation effects. The variational–perturbation method is based on the application of symmetry‐adapted pair functions taken in the form of partial‐wave (PW) expansions. The partial‐wave energy increments, pair energies, and total second‐order correlation energies E2 are calculated using extended radial basis sets. Special attention is paid both to the M and N intrashell and to the LM, LN, and MN intershell correlation effects. For every pair of the system all PW’s up to l′, l′′?9 are considered. Extrapolation of the pair energies for l′, l′′≳9 results in total second‐order energies EZn2+2 = −1.604 16 and EZn2 = −1.698 69 hartree. The systems considered, which contain the 3d10 and 3d104s2 electron configurations, are the largest for which a complete analysis of the correlation effects has been attempted by ’’ab initio’’ methods. The complicated structure of the systems offers the possibility of investigating various aspects of the electron correlation problem. The results confirm the suggestions made by Clementi [J. Chem. Phys. 42, 2783 (1965)] that for the outer shells of large atoms one is approaching the situation when a ’’complex pairing model’’ has to be used in place of the ’’simple pairing model’’ based on the notion of weak and strong electron pairs. Comparison with other related work is given.