Applications of coupled cluster theory
Turner, Walter Earl
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Ethylene is an exceptional example of a stable closed-shell singlet molecule with a low-lying triplet state of very different symmetry. The 3A1 state of ethylene, which has a twisted D2d geometry, is studied herein with coupled cluster theory. Geometric parameters are reported for C2H4, C2D4, and 13C2H4 . Harmonic and anharmonic vibrational frequencies are also predicted using second-order vibrational perturbation theory. Challenges encountered for the wagging vibrational features are discussed. Coupled cluster theory is further utilized to study the bonding, energetics, and vibrational frequecies of the the potential circumstellar molecule OPN. Isomers of OPN and several of its heavier group 15 and 16 congeners (SPN, SePN, TePN, OPP, OPAs, and OPSb) were examined. For OPN, OPP, and SPN, anharmonic vibrational frequencies and vibrationally corrected rotational constants are predicted; good agreement with available experimental data is observed.