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CASSCF/CCI calculations are presented for the low-lying states of Sc3 and Sc+3 and SCF/CI calculations are presented for the 1A’1 state of Ca3 arising from three ground state (4s2) Ca atoms, all for equilateral triangle geometries. The calculations use effective core potentials, developed by Hay and Wadt, which replace the Ne core but include the 3s and 3p core levels along with the valence electrons in the calculations. The bonding in Ca3 arises by 4s→4p promotion and leads to a well depth of about 0.5 eV for R(Ca–Ca)=7.5a0. For Sc3 the 4s bonding is similar to that in Ca3, but the 3d electrons are also strongly bonding leading to a 2A’2 ground state with a well depth of about 1.0 eV and R(Sc–Sc)=5.75a0. The good 3d bonding orbitals (bonding between all three atoms) are 3da″2 derived from atomic 3dπ″ and 3da1 derived from atomic 3dσ, while 3dπ’ atomic orbitals lead to 3de’ orbitals which are bonding between pairs of atoms, and the 3dδ’ and 3dδ″ derived levels are nonbonding. (Here the atomic symmetry is given with respect to an axis connecting the atom to the center of the molecule.) Based on the Sc3 calculations and preliminary calculations on Ti+3 , the bonding in V+3 and Cr+3 is also discussed. [ABSTRACT FROM AUTHOR]

Spectroscopic parameters (De, re, μ) are determined for the first-row transition metal hydrides using better than DZP basis sets at the modified coupled pair functional (MCPF) level. Extensive comparisons between MCPF and complete-active space self-consistent field (CASSCF)/MRCI calculations with natural orbital iterations, and studies with more extensive basis sets, show this level of treatment to supply an accurate and cost-effective treatment of these systems. For the transition metal hydrides, the bonding can arise from either the 3dn4s2 or 3dn+14s1 atomic asymptotes, or a mixture of both. Since the dipole moment arising from these two bonding mechanisms is very different, the dipole moment is found to be directly related to the 3d population. Thus, the magnitude of the dipole moments provide a sensitive test of the wave function, and gives insight into the nature of the bonding. [ABSTRACT FROM AUTHOR]

A calculated dipole moment of 2.39 D is reported for the X 2Δ state of NiH (based on extensive CASSCF/CI calculations). The computed dipole moment is found to be a sensitive function of the 3d population of the wave function; thus, the excellent agreement with the experimental dipole moment, 2.4±0.1 D, provides strong confirmation of the mixed state character of NiH (computed 3d population of 8.66 electrons). [ABSTRACT FROM AUTHOR]