Molecular orbital theory of the properties of inorganic and organometallic compounds 5. Extended basis sets for first‐row transition metals

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Abstract
A series of efficient split‐valence basis sets for first‐row transition metals, termed 3‐21G, has been constructed based on previously‐formulated minimal expansions of Huzinaga, in which each atomic orbital has been represented by a sum of three gaussians. The original Huzinaga expansions for s‐ and p‐type orbitals (except those for 1s) have been fit by least squares to new three‐gaussian combinations in which the two sets of orbitals (of the samenquantum number) share gaussian exponents. The Huzinaga three‐gaussian expansions for ls and 3d atomic orbitals have been employed without alteration. The valence description of the 3‐21G basis sets comprises 3d‐, 4s‐ and 4p‐type functions, each of which has been split into two‐ and one‐gaussian parts. 4p functions, while not populated in the ground state of the free atoms, are believed to be important to the description of the bonding in molecules. The performance of the 3‐21G basis sets is examined with regard to the calculation of equilibrium geometries and normalmode vibrational frequencies for a variety of inorganic and organometallic compounds containing first‐row transition metals. Calculated equilibrium structures, while generally superior to those obtained at STO‐3G, are not as good as those for compounds containing main‐group elements only. The calculations generally underestimate the lengths of double bonds between transition metals and main‐group elements, and overestimate the lengths of single linkages. Calculated normal‐mode vibrational frequencies for metal‐containing systems are less uniform than in those for main‐group compounds.

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