Basis set effects on spectroscopic constants for C2 and Si2 and the symmetry dilemma in the Xα model

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Abstract
In an attempt to perform thorough and accurate linear combination of atomic orbitals (LCAO) Xα, calculations on C2 and Si2, the effects of basis set composition on the computed spectroscopic constants are investigated for the first time. The questions of how to tell if an adequate basis set has been found and what Xα state corresponds to the lowest state of 1Σ+g symmetry which involves strong electron correlation effects are addressed. With the larger bases, general agreement with experiment and previous density functional calculations are obtained. Remarkable agreement with the configuration interaction (CI) calculations of Bruna et al. on Si2 is obtained if the fractional occupation number method of Slater et al. is used.