Chemical Physics Letters, 844, 141259 (2024)
Calculation of level densities of coupled anharmonic molecular vibrations
Rui Zhang1,2, Klavs Hansen3,∗, John W. Niman4, Piero Ferrari5, Shimpei Iida6 and Haruo Shiromaru6
1 School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China.
2 Department of Physics, Tsinghua University, Beijing 100084, China.
3 Center for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China.
4 Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-0484, USA.
5 Radboud University, Institute for Molecules and Materials, HFML-FELIX, Nijmegen, Netherlands.
6 Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan.
* klavshansen@tju.edu.cn
Abstract
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. C7- is used as a case study.
