Interactions of transition metal atoms in high-spin states: Cr2, Sc-Cr, and Sc-Kr.

The high-spin van der Waals states are examined for the following dimers: Cr(2) ((13)Sigma(g)(+)), Sc-Cr ((8)Sigma(+), (8)Pi, (8)Delta), and Sc-Kr ((2)Sigma(+), (2)Pi, (2)Delta). These three systems offer a wide range of van der Waals interactions: anomalously strong, intermediate, and typically weak. The single-reference [coupled cluster with single, double, and noniterative triple excitations, RCCSD(T)] method is used in the calculations for all three systems. In addition, a range of configuration-interaction based methods is applied in Cr(2) and Sc-Cr. The three dimers are shown to be bound by the dispersion interaction of varying strength. In a related effort, the dispersion energy and its exchange counterpart are calculated using the newly developed open-shell variant of the symmetry-adapted perturbation theory (SAPT). The restricted open-shell time-dependent Hartree-Fock linear response function is used in the calculations of the dispersion energy in Sc-Cr and Sc-Kr calculations, while the restricted open-shell time-dependent density functional linear response function is used for Cr(2). A hybrid method combining the repulsive restricted open-shell Hartree-Fock (or complete active space self-consistent field) interaction energy with the dispersion and exchange-dispersion terms is tested against the RCCSD(T) results for the three complexes. The Cr(2) ((13)Sigma(g)(+)) complex has the well depth of 807.8 cm(-1) at the equilibrium distance of 6.18a(0) and the dissociation energy of 776.8 cm(-1). The octet-state Sc-Cr is about four times more strongly bound with the order of well depths of (8)Delta>(8)Pi>(8)Sigma(+) and a considerable anisotropy. The enhanced bonding is attributed to the unusually strong dispersion interaction. Sc-Kr ((2)Sigma(+), (2)Pi, (2)Delta) is a typical van der Waals dimer with well depths in the range of 81 cm(-1) ((2)Delta), 84 cm(-1) ((2)Sigma(+)), and 86 cm(-1) ((2)Pi). The hybrid model based on SAPT leads to results which are in excellent qualitative agreement with RCCSD(T) for all three interactions.