Assessment of Multi-Scale Approaches for Computing UV-Vis Spectra in Condensed Phases: Toward an Effective yet Reliable Integration of Variational and Perturbative QM/MM Approaches.

Computational simulation of UV/vis spectra in condensed phases can be performed starting from converged molecular dynamics (MD) simulations and then performing quantum mechanical/molecular mechanical (QM/MM) computations for a statistically significant number of snapshots. However, the need of variational solutions (e.g., ONIOM/EE) for a huge number of snapshots makes unpractical the use of state-of-the-art QM Hamiltonians. On the other hand, the effectivity of perturbative approaches (e.g., perturbed matrix method, PMM) comes at the price of poor convergence for configurations strongly different from the reference one. In this paper we introduce an integrated strategy based on a cluster analysis of the MD snapshots. Next, a representative configuration for each cluster is treated at the ONIOM/EE level, whereas local fluctuations within each cluster are described at the PMM level. Some representative systems (uracil in dimethylformamide and in water and tyrosine zwitterion in water) are analyzed to show the effectivity and flexibility of the proposed strategy.