On-the-fly ab initio semiclassical evaluation of third-order response functions for two-dimensional electronic spectroscopy

TL;DR

This paper introduces a computationally efficient semiclassical method for accurately calculating two-dimensional electronic spectra, accounting for harmonic and some anharmonic effects, improving upon simpler models.

Contribution

It presents a new single-trajectory semiclassical approach that is exact for harmonic potentials and partially accounts for anharmonicity, enhancing spectral simulations.

Findings

The method accurately captures Duschinsky and frequency change effects.

Anharmonicity effects are weak in phenol, but Duschinsky effects are significant.

Displaced harmonic oscillator models are insufficient for detailed spectral features.

Abstract

Ab initio computation of two-dimensional electronic spectra is an expanding field, whose goal is improving upon simple, few-dimensional models often employed to explain experiments. Here, we propose an accurate and computationally affordable approach, based on the single-trajectory semiclassical thawed Gaussian approximation, to evaluate two-dimensional electronic spectra. Importantly, the method is exact for arbitrary harmonic potentials with mode displacement, changes in the mode frequencies, and inter-mode coupling (Duschinsky effect), but can also account partially for the anharmonicity of the involved potential energy surfaces. We test its accuracy on a set of model Morse potentials and use it to study anharmonicity and Duschinsky effects on the linear and two-dimensional electronic spectra of phenol. We find that in this molecule, the anharmonicity effects are weak, whereas the Duschinsky rotation and the changes in the mode frequencies must be included in accurate simulations. In contrast, the widely used displaced harmonic oscillator model captures only the basic physics of the problem but fails to reproduce the correct vibronic lineshape.