Modeling vibrational anharmonicity in infrared spectra of high frequency vibrations of polyatomic molecules

In this perspective, we review the challenges of calculating spectra of high-frequency XH vibrations (where X = C, N, or O) of molecules and small clusters. These modes are often coupled to nearly degenerate overtone and combination bands, greatly complicating the interpretation of the spectra. When molecules or clusters contain multiple XH groups, assigning spectra is difficult, especially when multiple conformers are present. We consider approaches appropriate for addressing these difficulties, focusing on systems with more than 15 atoms. At this size, the densities of states of these fundamentals are sufficiently high that it is not possible to calculate eigenstate-resolved spectra. Nonetheless, combining perturbation theory, empirical scalings of vibrational frequencies, and variational treatments of reduced dimensional Hamiltonians, one can identify and model the vibrational coupling pathways that influence observed spectral features. We describe how these methods have evolved through time as electronic structure methods and computational resources have advanced.