Improving PCM in Protic Media: Markov State Models for TD-DFT Calculations

Predicting the optical properties of dyes in protic media requires accounting for specific solute-solvent interactions, which go beyond polarizable continuum models (PCM). We introduce a workflow that combines molecular dynamics simulations, time-lagged independent component analysis, and Markov State Models (MSM) to identify key macrostates and system configurations relevant to model electronic excitations using time-dependent density functional theory (TD-DFT) with only H-bonded solvent molecules in the explicit input. This MSM(PCM)TD-DFT approach shows that just a few thermally accessible, nonoptimized geometries could be more representative of the experimental spectra than optimized structures. By testing this approach on two compounds, including one with a transition with strong charge-transfer character, we show that MSM(PCM)TD-DFT is a promising way to understand these kinds of systems in a cost-effective way.