Two-dimensional vibrational spectroscopy provides invaluable insight into molecular structure and dynamics of condensed-phase systems. Specifically, intermolecular interactions in liquids and solutions can be probed by hybrid THz/Raman techniques, which involve two ultrashort terahertz pulses and one near-infrared or visible (Raman) pulse. Such techniques were recently used to study how nuclear quantum effects, arising due to the quantum-mechanical nature of light atomic nuclei, affect structural inhomogeneity of liquid water. However, corroborating experimental conclusions with theory is difficult due to the high computational cost of quantum dynamical simulations. Here, we propose a practical computational method called equilibrium-nonequilibrium hybrid ring-polymer molecular dynamics, which can include nuclear quantum effects on two-dimensional THz/Raman spectra and is exact in the classical limit, where it reduces to the classical molecular dynamics method of Tanimura and coworkers. Like its classical analogue, the ring-polymer molecular dynamics approach requires one equilibrium and two nonequilibrium trajectories for each initial condition sampled from the quantum thermal distribution. Hence, the computational cost of evaluating complex and information-rich THz/Raman spectra is only three times the cost of evaluating standard one-dimensional vibrational (e.g., infrared absorption or Raman) spectra. We demonstrate the advantages of this new method by comparing it with classical and exact quantum approaches on model systems. Furthermore, we apply equilibrium-nonequilibrium ring-polymer molecular dynamics to study how nuclear quantum effects in liquid neon and water affect their two-dimensional spectra.

Equilibrium-nonequilibrium hybrid ring-polymer molecular dynamics for two-dimensional vibrational spectroscopy

ACS Spring 2022

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