Two-dimensional spectra across the electromagnetic spectrum probe interactions among coupled molecular degrees of freedom. Building a molecular picture from measured spectra is assisted by dynamical modeling of the measurement. For two-dimensional infrared (2DIR) spectroscopy, this modeling requires performing quantum dynamics of nucleii on a single electronically adiabatic potential surface, while for measurements with one dimension each corresponding to vibrational and vibronic frequencies such as two-dimensional vibrational-electronic (2DVE) spectroscopy, the computation involves multiple electronic surfaces. The Optimized Mean Trajectory (OMT) method is a semiclassical approximation to quantum dynamics allowing nonlinear spectroscopic response functions to be computed from ensembles of continuous classical trajectories interrupted by discontinuities representing the radiation-matter interaction. For a 2DIR calculation, these are nuclear trajectories on a single electronic surface. For a calculation including vibronic transitions, the nuclear degrees of freedom are supplemented by classical mapping variables representing electronic states.<br/>The OMT approach performs well in calculations of 2DIR spectra of solvated anharmonic vibrations undergoing both energy relaxation and pure dephasing processes. The method also captures 2DVE spectra for a system including excitonically coupled electronic excited states and a chromophore vibration interacting with a resonant solvent. Given potential surfaces and couplings, the OMT approximation allows the efficient calculation of multidimensional vibrational and vibronic spectra from classical dynamics.

Calculating two-dimensional vibrational and vibrational-electronic spectra from classical trajectories

ACS Fall 2021

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