Bridging atomistic accuracy and computational efficiency in condensed-phase spectroscopy via the multistate harmonic model

Simulating electronic spectroscopy in the condensed phase requires capturing the interplay of nonadiabatic dynamics, vibronic couplings, and environmental correlations. While direct all-atom quantum dynamics can provide rigorous benchmarks, they are computationally expensive due to the high dimensionality of the solvent environment. Here, we demonstrate that the Multistate Harmonic (MSH) model serves as an accurate and highly efficient surrogate for the full atomistic Hamiltonian. By mapping the energy-gap fluctuations of the organic photovoltaic acceptor Y6 in chloroform onto a globally shared bath, the MSH model reduces the system complexity from over 8200 atoms to ∼100 effective modes while preserving essential environmental correlations. We show that nonadiabatic semiclassical mapping dynamics performed on this reduced MSH Hamiltonian accurately reproduce the linear absorption spectra and ultrafast population dynamics obtained from full all-atom simulations, with a computational cost reduction of over 97%. This framework offers a robust and scalable protocol for predicting spectroscopic signals in complex condensed-phase systems.