Research in the Li group focuses on developing and applying electronic structure theories and ab initio molecular dynamics for studying properties and reactions, in particular non-adiabatic reactions that take place in large systems, such as polymers, biomolecules, and clusters. Students will have a unique opportunity to participate in interdisciplinary research subjects.
The direct simulation of the time evolution of a chemical system is a powerful tool and a valuable companion to experimental measurements. The proper description of dynamical processes such as laser induced chemistry and electron transfer in molecular, biological, and nanomaterial systems requires going beyond the perturbative regime of linear-response theories and the adiabatic approximations of Born-Oppenheimer and extended Lagrangian based molecular dynamics. With this in mind, our group has worked to develop multiple time-domain techniques for the description of electronic and nuclear dynamics. Our implementation of Ehrenfest dynamics within time-dependent Hartree-Fock (TDHF) and time-dependent density functional theory (TDDFT) has been used to study a variety of exciting processes, including laser-controlled dissociation of molecules, modeling doubly excited states, and intramolecular charge transfer in a fullerene derivative. Recently, we have also worked on developing our own approaches to open-system electronic dynamics, as well as, a new hybrid of trajectory surface hopping and Ehrenfest dynamics for the efficient simulation of non-adiabatic molecular dynamics.
Doping inorganic semiconductor quantum dots (QDs) with transition-metal ions introduces unique electronic, optical, and magnetic properties that are otherwise absent in the host semiconductor lattice. The potential to integrate room-temperature ferromagnetism with the electrical properties of semiconducting materials makes these so-called diluted magnetic semiconductors (DMSs) attractive for spin-electronic and spin-photonic applications. Our recent advances have revealed the microscopic origins of carrier-dopant and dopant-dopant exchange interactions and the effects of such interactions on the absorption spectra of these materials.
The overarching objective of this program is to computationally design and search for stable and controllable nano-sized DMS materials. Specifically, our research focuses on (i) the roles of defects, such as transition-metal and p-type dopants, in activating high Curie-temperature ferromagnetism; (ii) charge-transfer transitions for potential solar energy conversion; (iii) chemical and physical processes required to control magnetization using time-dependent quantum mechanical electrodynamics of charge-transfer delocalization, magnetic polaron formation, and magnetization reversal dynamics; and (iv) dopant-centered Auger-type processes and their role in the photodynamics of these doped nanocrystals.
Computational methods are applied to large systems of ever increasing size. Accurate modeling and analysis of biomolecules, polymers and nanostructures with hundreds to thousands of atoms are often computationally demanding because of excessive degrees of freedom. Improvement in minimizing the computational cost and enhancement to the general stability of structure optimization and conventional algorithms would be highly appreciated.
We are actively developing efficient optimization methods and algorithms in areas related to electronic wave function and nuclear geometry optimizations, electronic optical response, time-dependent solvent effects, and linear scaling of two-electron integration. We are also interested in applying the computational methods of quantum chemistry to advanced computer machinery involving cloud computing techniques, high performance parallelism and cost effective sparse matrix manipulations.