Research directions focus on the theoretical description and simulation of quantum dynamical phenomena in systems of varying complexity, ranging from solute-solvent systems to chromophore-protein and chromophore-RNA complexes, to semiconducting polymers. While elementary quantum dynamical processes in small molecular systems have been analyzed in some detail over the past few decades, the interplay of such processes with structured and dynamically responding environments poses a considerable challenge. This is particularly manifest in ultrafast excited-state processes, where coherent quantum evolution is accompanied by the environment's non-equilibrium dynamics. Examples from our recent work include the photophysics of organic semiconductor polymers, involving, in particular, exciton decay processes at donor-acceptor polymer heterojunctions [Tamura, Burghardt, J. Am. Chem. Soc. (Communication) 135, 16364 (2013), Huix-Rotllant, Tamura, Burghardt, J. Phys. Chem. Lett. 6, 1702 (2015)], and singlet fission [Tamura, Huix-Rotllant, Burghardt, Olivier, Beljonne, Phys. Rev. Lett. 115, 107401 (2015)], as well as the controlled destabilization of RNA and DNA by azobenzene photoswitches [Mondal, Biswas, Goldau, Heckel, Burghardt, J. Phys. Chem. B 119, 11275 (2015)].
Our approach combines (i) quantum and mixed quantum-classical methods suitable for many dimensions, in particular multiconfigurational methods and trajectory-based methods, with (ii) reduced-dimensionality models, for example effective-mode models [Martinazzo, Hughes, Burghardt, Phys. Rev. E (Rapid Communication) 84, 030102(R) (2011)]. Current efforts are directed, in particular, at multiconfigurational Gaussian-based representations in many dimensions [Roemer, Ruckenbauer, Burghardt, J. Chem. Phys. 138, 064106 (2013), Richings et al., Int. Rev. Phys. Chem. 34, 265 (2015)]. A complementary strategy focuses on mesoscopic descriptions like classical dynamical density functional theory (DDFT) for the description of nonequilibrium solvation. In [Burghardt, Bagchi, Chem. Phys. 329, 343 (2006), Hughes, Baxter, Bousquet, Ramanathan, Burghardt, J. Chem. Phys. 136, 014102 (2012)], we have proposed a new mixed quantum-classical approach which couples quantum molecular coordinates to a "generalized solvent coordinate", using a classical hydrodynamic description of the solvent. This hybrid approach provides a promising tool for the study of elementary photochemical events in solution.