Calculation of level densities of coupled anharmonic molecular vibrations

We present a method to calculate molecular vibrational densities of states (level densities) with anharmonic vibrations, crucial in many cases for describing thermal properties of molecular species. Coupling of modes is fully implemented and the method eliminates the severe numerical artefacts associated with the commonly used limitation to the second order expansion in quantum numbers with their negative coupling constants. The method proceeds by calculating the canonical partition function perturbatively and applying the inverse Laplace transform. C-7(-) is used as a case study.