Higher-order electron correlation effects in triatomic potential energy surfaces. Stretching vibrations of HCN and HNC

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Abstract
An assessment of the importance of higher-order electron correlation effects on harmonic vibrational frequencies, anharmonicities, and equilibrium structures is made for the simple triatomics HCN and HNC. Potential energy surfaces for bond stretching were generated with a large, polarized basis set of contracted Gaussian functions and using self-consistent field (SCF), correlated self-consistent electron pair (SCEP), and approximate double substitution coupled cluster (ACCD) wave functions. SCEP includes correlating configurations that mix directly with the reference wave function, while ACCD includes these configurations and also indirectly mixing configurations that arise from a cluster model of the wave function. The surfaces are compared in the prediction of spectroscopic parameters by using a low-order perturbational treatment of the vibrational wave functions in a basis of normal mode functions and with several polynomial surface fits. The neglect of correlation effects at the SCF level leads to overestimates of the vibrational frequencies. Higher-order correlation effects correct for roughly one third of this overestimation. For all parameters, the ACCD treatment yields the best predictions.