Approximate inclusion of four-mode couplings in vibrational coupled-cluster theory

作者：Zoccante Alberto^*; Seidler Peter; Hansen Mikkel Bo; Christiansen Ove

来源：Journal of Chemical Physics, 2012, 136(20): 204118.

DOI：10.1063/1.4721626

The vibrational coupled cluster (VCC) equations are analyzed in terms of vibrational Moller-Plesset perturbation theory aiming specifically at the importance of four-mode couplings. Based on this analysis, new VCC methods are derived for the calculation of anharmonic vibrational energies and vibrational spectra using vibrational coupled cluster response theory. It is shown how the effect of four-mode coupling and excitations can be efficiently and accurately described using approximations for their inclusion. Two closely related approaches are suggested. The computational scaling of the so-called VCC[3pt4F] method is not higher than the fifth power in the number of vibrational degrees of freedom when up to four-mode coupling terms are present in the Hamiltonian and only fourth order when only up to three-mode couplings are present. With a further approximation, one obtains the VCC[3pt4] model which is shown to scale with at most the fourth power in the number of vibrational degrees of freedom for Hamiltonians with both three-and four-mode coupling levels, while sharing the most important characteristics with VCC[3pt4F]. Sample calculations reported for selected tetra-atomic molecules as well as the larger dioxirane and ethylene oxide molecules support that the new models are accurate and useful.

• 出版日期2012-5-28