AnhDis: A Scheme for Anharmonic Nuclear Ensembles and Absorption Spectra
Year: 2026
Type: preprint
Abstract: The Nuclear Ensemble Approach (NEA), typically implemented using the harmonic Wigner approximation, has shown significant limitations when applied to molecules with pronounced anharmonicity. The harmonic model frequently samples unphysical regions of the potential energy surface, resulting in artificial spectral broadening and spurious transitions. In this work, the Anharmonic-Distribution (AnhDis) scheme is presented. The method is based on constructing mode-resolved potential energy surfaces through explicit scans, represented in the present implementation by quartic polynomial fits, although higher-order expansions or other one-dimensional potential energy functions can be employed when required. By solving the resulting one-dimensional vibrational Hamiltonians, physically consistent nuclear distributions that account for anharmonic confinement and wave functions asymmetry are obtained. Temperature effects are incorporated through Boltzmann weighting of the vibrational eigenstates. AnhDis significantly improves upon harmonic sampling while maintaining the computational feasibility required for large-scale systems. Validation against higher-level reference approaches and experimental data across atmospherically relevant molecules demonstrates that the proposed method captures absorption spectral profiles while reducing harmonic-induced artifacts.
Source: ChemRxiv
Institution: Universitat de València
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220420259