Force constants for the symmetric stretch motions of acetylene: Accurate a b i n i t i o calculations

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Abstract
Quadratic force constants for C2H2 have been determined from D∞h potential surfaces obtained using SCF and configuration interaction, CI, wavefunctions. Our best calculation yields quadratic force constants which are of the same accuracy as those derived from experimental data. This calculation is based on a CI model which includes single and double exctitations of all valence electrons and which uses an extended Gaussian basis set with several polarization functions. We have performed calculations using different numbers of polarization functions in the basis set and correlating different numbers of electrons in the CI wavefunctions. The sensitivity of the computed equilibrium geometry and quadratic force constants to these variations have been investigated.