Electron pair density in the lowest 1Σu+ and 1Σg+ states of H2

Wang, Jian; Kim, Kwang S.; Baerends, Evert Jan
August 2011
Journal of Chemical Physics;8/21/2011, Vol. 135 Issue 7, p074111
Academic Journal
We demonstrate and advocate the use of observable quantities derived from the two-electron reduced density matrix - pair densities, conditional densities, and exchange-correlation holes - as signatures of the type of electron correlation in a chemical bond. The prototype cases of the lowest 1Σu+ and 1Σg+ states of H2, which exhibit large variation in types of bonding, ranging from strongly ionic to covalent, are discussed. Both the excited 1Σg+ and 1Σu+ states have been interpreted as essentially consisting of (natural) orbital configurations with an inner electron in a contracted 1sσg orbital and an outer electron in a diffuse (united atom type, Rydberg) orbital. We show that nevertheless totally different correlation behavior is encountered in various states when comparing them at a common internuclear distance. Also when following one state along the internuclear distance coordinate, strong variation in correlation behavior is observed, as expected. Switches between ionic to covalent character of a state occur till very large distances (40 bohrs for states approaching the 1s3ℓ asymptotic limit, and 282 bohrs for states approaching the 1s4ℓ limit).


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