Our research interests focus on theoretical studies of chemical dynamics in complex molecular systems such as liquids, surfaces, molecular aggregates, and biological molecules. The goal is to provide a description of fundamental chemical processes in such systems at the atomistic detail. To achieve this goal, we apply the theory of quantum dynamics and statistical mechanics, aided by computer simulations using modern quantum chemistry and molecular dynamics packages. We are particularly interested in developing new theoretical tools to understand chemical processes where the quantum effect of nuclear dynamics is of the central importance, such as electron and proton transfer reactions, and excited state dynamics.

 

(1) Theoretical methods for quantum dynamics in complex systems. Our theoretical tools to study the multi-dimensional quantum dynamics in complex systems are semiclassical methods and the generalized quantum master equation methods. We are developing more accurate and efficient methods, as well as new numerical algorithms that allow us to study large systems. Recently, we have developed an efficient algorithm to solve the non-perturbative and non-Markovian hierarchical equations of motion (HEOM), which have been applied in simulations of charge and energy transfer reactions, as well as time resolved nonlinear spectroscopy. We are also developing efficient mixed quantum-classical method to simulation non-adiabatic dynamics in condensed phases.

 

(2) Electron and Proton transfer reactions. Redox and acid-base reactions are ubiquitous in chemical processes in solution and biological systems. Their pronounced quantum nature is due to the light mass of electrons and protons, which often gives rise to quantum tunneling and zero-point energy effects. Effects of dephasing and relaxation also play an important role in these processes. Using a double well model coupled to a harmonic bath, we have investigated proton transfer reactions from the deep tunneling to over the barrier regimes, as well as the effect of the rate promoting vibrational modes on the proton transfer. We are also working on proton coupled electron transfer reactions in solution and biological systems.

 

(3) Excited state dynamics and theoretical spectroscopy. Excited state dynamics is important in photophysics and photochemical processes, and nonlinear optical spectroscopy techniques are important tools to probe such dynamics. Quantum effects are essential in these studies since the dynamics involves simultaneous motion on several potential surfaces. We have simulated various types of linear and non-linear spectroscopic signals of photosynthetic light harvesting complexes, which give important information on excitation energy delocalization, energy transfer and relaxation, as well as quantum coherence. We are also interested in vibrational dynamics such as vibrational energy relaxation and dephasing, IR and Raman spectroscopy.