A VPT2 route to near-infrared spectroscopy: The role of mechanical and electrical anharmonicity

Following the previous developments to simulate fully anharmonic spectra within the vibrational second-order perturbation level of theory [ J. Chem. Phys. 2012, 126, 134108 ], an extension to transitions up to three quanta is presented here. A general formulation including the mechanical and electrical anharmonicities is adopted to facilitate the support of additional properties, and thus spectroscopies. In addition to providing more accurate theoretical band shapes, inclusion of overtones and combination bands up to three quanta paves the way to a more complete interpretation of near-infrared spectra.