## Anharmonic Vibrational Properties from Intrinsic *n*‑Mode State Densities

journal contribution

posted on 16.12.2015, 23:15 by Eugene Kamarchik, Ahren W. JasperA method for calculating fully anharmonic
vibrational state counts,
state densities, and partition functions for molecules is presented.
The method makes use of a new quantity, the

*intrinsic*density of states, which is associated with the states that uniquely arise from a given mode, mode pairing, or higher-order mode coupling. By using only low-order intrinsic densities, the fully coupled anharmonic vibrational result can be constructed, as shown by our application of the method to methane, CH_{4}, and cyclopropene, C_{3}H_{4}. Truncation of the intrinsic expansion at the coupling of pairs of modes yields greatly improved scaling over direct evaluation of the full-dimensional result and recovers a large fraction of the total anharmonicity. We also discuss the relation of the new quantities to the structure of the potential energy surface.