Binding Energy Curves from Nonempirical Density Functionals II. van der Waals Bonds in Rare-Gas and Alkaline-Earth Diatomics

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Abstract
Binding energy curves have been calculated for the ground-state rare-gas diatomics Ne2 and Ar2 and for the alkaline-earth diatomic Be2 using the nonempirical density functionals from the first three rungs of a ladder of approximations: the local spin density (LSD) approximation, the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA), and the Tao−Perdew−Staroverov−Scuseria (TPSS) meta-GGA. Binding energy curves in reasonable agreement with those constructed from experiment are found from PBE and TPSS, which incorporate inhomogeneity corrections that satisfy the Lieb−Oxford bound and so describe the short-range part of the van der Waals interaction. At large internuclear separation, these functionals produce an exponentially decaying attraction in place of the correct long-range −C6/R6. Basis-set and exchange-only effects are also discussed.