Accurate Theoretical Determinations of Vibrational Frequencies and Zero-Point Vibrational Energies for Molecules

This work aims at exploring a theoretical method to predict the vibrational frequencies of molecular species at high precision. Several methods that take account of the anharmonicity, and thus go beyond the usual harmonic approximation, are considered: vibrational self-consistent-field (VSCF) and its variants [vibrational configuration interaction (VCI), vibrational perturbation theory (VPT) and vibrational coupled cluster (VCC)], the second-order perturbative treatment of anharmonicity, and the transition-optimized shifted Hermite (TOSH) theory. The researchers will compare the computed anharmonic vibrational frequencies from various methodologies with the existing experimental frequencies and carry out detailed statistical analysis on the performance of these methods. With this work, they will investigate several possibilities for extracting accurate zero-point vibrational energy (ZPVE) information from the computed anharmoncic vibrational frequencies of the molecules. Such accurate ZPVE correction must be incorporated into current state-of-the-art theoretical methods to improve the accuracy in energetic and thermochemistry predictions.