Modern time-resolved spectroscopy and X-ray scattering techniques are sensitive to transient electronic and structural rearrangements of molecules in solution. However, the interpretation and analysis of ultrafast experimental data is often complicated. Molecular dynamics simulations are needed to support the experiments and provide insights into the influence of the ultrafast processes on, e.g., photocatalysis and energy conversion. We develop direct dynamics simulation methodologies combining variational density functional theory (DFT) calculations of excited electronic states with an explicit QM/MM description of solvent effects. As an example application, it will be shown how Born-Oppenheimer molecular dynamics simulations using QM/MM electrostatic embedding can elucidate the influence of the nonequilibrium solvent response on the photoinduced ligand dynamics in a copper molecular photosensitizer.<br/><br/>Typically, the photoinduced dynamics are nonadiabatic, requiring approaches beyond the Born-Oppenheimer approximation, but self-consistent field methods commonly employed within DFT tend to fail when electronic degeneracies are present. To address this issue, we have recently developed novel algorithms based on direct orbital optimization that can more robustly converge ground- and excited-state solutions. Examples will be shown where these methods are used to obtain potential energy surfaces in challenging cases, including a doubly excited charge-transfer state and a conical intersection in ethylene, and curve crossings between metal-to-ligand charge-transfer and metal-centered states in an iron molecular photosensitizer. These new variational DFT approaches can additionally be combined with the QM/MM method. Therefore, they pave the way for efficient and reliable exploration of potential energy surfaces within molecular dynamics simulations including nonadiabatic electronic transitions and nonequilibrium solvation effects.

Modelling photoinduced molecular dynamics with solvation effects via QM/MM and excited-state orbital optimized density functional calculations

ACS Spring 2022

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