Vibrational perturbation theory (VPT) has undergone an Renaissance over the past decade, replacing scaled harmonic treatments as the general method of choice for the evaluation of vibrational spectra. John Stanton and his group have made significant contributions in this area, and in the broadening of the range of calculations that can be performed using VPT. In this talk, we will present recent work in our group in the development of a numerical implementation of VPT that is based on the work of Sakurai. This implementation allows us to explore the influence of the choice of coordinates in which the Hamiltonian is expanded on the resulting energies and intensities. It also allows us to consider transitions to states with three or four quanta of excitiation; spectra originating from excited states and to take the perturbation theory to higher orders. Of particular interest is the ability to dissect contributions from the potential (e.g. mechanical anharmonicity) and the dipole surface (e.g. electrical anharmonicity) to the intensity of transitions to states with two or more quanta of excitation, which would expected to be dark based on the harmonic oscillator/linear dipole treatment of vibrational spectra.

Exploring origins of anharmonicity using vibrational perturbation theory (VPT)

ACS Spring 2022

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