Theoretical Chemical Dynamics. In recent years, advances in both experiment and theory have allowed an increasingly detailed understanding of elementary chemical processes in the gas phase. Much less is known, however, about chemical dynamics and reactivity in complex many-body systems. The general goal of our research program is to extend the detailed understanding currently attainable for few-body systems to many-body problems.

Ultrafast Processes on the Nanoscale. We are interested in the phenomenology of ultrafast dynamical processes in complex systems--the detailed course of events on atomic length scales and femtosecond to picosecond time scales where dynamical "decisions" are made: sudden energy transfer events, bond breakage or formation, barrier crossing or recrossing, and others. The physical problems we consider span a wide range of phenomena, including chemical reaction dynamics in van der Waals clusters, photochemistry of molecules in clusters and solids, the control of chemical processes by shaped laser pulses, and dynamical processes in energetic materials. The theoretical methods we employ to study these problems include classical molecular dynamics simulations, quantum mechanical wave packet propagation, and classical-quantum hybrid approaches. Due to the vast amount of numerical data produced in many-body simulations, visualization and data analysis are important components of our research. Ultimately, we strive to develop simple analytic models of the processes revealed.

The Classical-Quantum Frontier. Classical mechanics forms the basis of our intuition about how the universe operates and provides an easy and efficient way to calculate molecular dynamics on the computer. However, manifestly quantum mechanical phenomena, such as transitions between coupled electronic states, electronic coherence and its decay, or quantum mechanical tunneling, require fundamental modification of the purely classical motion. Nonclassical phenomena play a key role in 21^st Century technologies such as molecular electronics, nanotechnology, quantum computing, and quantum cryptography. We are currently exploring the frontier between classical and quantum mechanical theories of molecular dynamics by developing approaches to solving quantum equations of motion using ensembles of classical trajectories. The resulting methods are applied to molecular problems, including nonadiabatic dynamics, coherent multistate electronic-nuclear dynamics, and tunneling through potential barriers. When viewed from this "quantum trajectory ensemble" perspective, quantum effects arise in a novel and intriguing way: as a breakdown of the statistical independence of the trajectories in the ensemble and a nonlocal entanglement of their collective evolution.