An accurate and efficient Ehrenfest dynamics approach for calculating linear and nonlinear electronic spectra

TL;DR

This paper introduces a pure state Ehrenfest method for accurately computing linear and nonlinear electronic spectra, improving upon previous approaches especially for complex systems and initial coherences.

Contribution

The paper develops a pure state Ehrenfest approach that enhances accuracy in calculating spectra for large, complex systems with excited state coherences.

Findings

Accurately reproduces linear, 2DES, and pump-probe spectra in slow bath regimes.

Captures main spectral features even in fast bath regimes.

Shows significant accuracy improvements over projected Ehrenfest methods.

Abstract

Linear and nonlinear electronic spectra provide an important tool to probe the absorption and transfer of electronic energy. Here we introduce a pure state Ehrenfest approach to obtain accurate linear and nonlinear spectra that is applicable to systems with large numbers of excited states and complex chemical environments. We achieve this by representing the initial conditions as sums of pure states and unfolding multi-time correlation functions into the Schr\"odinger picture. By doing this we show that one can obtain significant improvements in accuracy over the previously used projected Ehrenfest approach and that these benefits are particularly pronounced in cases where the initial condition is a coherence between excited states. While such initial conditions do not arise when calculating linear electronic spectra, they play a vital role in capturing multidimensional spectroscopies. We demonstrate the performance of our method by showing that it is able to quantitatively capture the exact linear, 2DES, and pump-probe spectra for a Frenkel exciton model in slow bath regimes and is even able to reproduce the main spectral features in fast bath regimes.