Vibrational analysis of small molecules at the complete basis set limit

Vibrational analysis is an ab initio technique that explains the infrared spectrum of a molecule by describing its internal motion. Spectroscopists and astronomers rely on this information to explore molecular vibrations observed in the laboratory or in the interstellar medium of space. In this work, we showcase computational chemistry methods that are able to predict anharmonic vibrational frequencies to sub-wavenumber, or spectroscopic, accuracy. We benchmarked this impressive capability with formaldehyde before applying similar methods to other small molecules of interest. The first is hydrogen sulfide cation; which has yet to be observed in space but is a likely contributor to interstellar sulfur depletion. Another important astronomical molecule, cyclopropenylidene, is then critically analyzed due to recent challenges in its theoretical description. Lastly, the fundamental vibrations and energy level splittings of the elusive methyl anion are explored with high-level methods.