Anharmonic spectroscopic features in quasiclassical dynamics: the importance of the quantum initial conditions

Practical approaches for the simulation of quantum nuclear dynamics often combine sampling of a set of quantum initial conditions with their (quasi)classical time-evolution. Observable properties, in particular spectra, are then computed from appropriate averages over these trajectories. In this talk, we combine a perturbative analysis with numerical calculations to examine more in detail the performance of different quasiclassical approaches to capture overtones, combination bands, and Fermi resonances in model systems of increasing complexity. We show that the recently introduced Edgeworth approximation for the sampling of the Wigner quantum density [1] allows capturing these anharmonic spectroscopic features accurately (exactly at lowest perturbation order). Our results highlight the importance of capturing some inherently quantum aspects of the Wigner distribution (in particular position-momentum correlations) in order to reproduce finer spectral features which are often considered out-of-reach of quasiclassical methods due to their lack of quantum coherence effects.

[1]T. Plé, S. Huppert, F. Finocchi, P. Depondt, and S. Bonella, “Sampling the thermal Wigner density via a generalized Langevin dynamics”, J. Chem. Phys. 151, 114114 (2019)