Harmonic vibrational frequencies and infrared intensities from analytic fourth-order many-body perturbation theory gradients

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Abstract
Recently developed fourth-order many-body perturbation theory [MBPT(4)] analytic gradient techniques are applied to a study of the harmonic vibrational frequencies and infrared intensities of prototype small polyatomic molecules. Results are presented for the complete fourth-order model as well as for the SDQ-MBPT(4) scheme, which neglects the contribution of triple excitations. These results are also compared to frequencies and intensities obtained at both higher and lower levels of theory. Differences between the fourth-order results and those obtained with the infinite order coupled-cluster (CC) counterparts of SDQ-MBPT(4) and MBPT(4) [CCSD and CC models including triple excitations, the latter approximated here by CCSDT-1 and the UCC(4) model] are found to be negligible for all bending modes and stretches involving single bonds. However, the infinite order effects included in CC models are important for describing stretching modes of multiple bonds. Therefore, iterative CC calculations are needed to accurately characterize these regions of the potential surface.

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