Harry and Catherine Jaynne Boand Endowed Professor of Chemistry
co-Associate Chair for Graduate Education
Ph.D. Wayne State University, 2003
Research in the Li group focuses on the development of time-dependent electronic structure theory, relativistic quantum mechanical techniques, and new methods for studying non-adiabatic dynamics of large-scale systems.
Time-Dependent Many-Body Theory and Relativistic Electronic Structure Theory
The time-dependent Schrödinger and Dirac equations govern all non-equilibrium quantum mechanical processes of a many-electron system. Understanding these processes is crucial to many advanced scientific research and technological developments. One area of the research effort in the Li group focuses on the development of time-dependent many-electron theories and computational methods that can be applied to simulate realistic and chemically relevant non-equilibrium many-electron phenomena driven by external perturbations (electromagnetic fields and dielectric media) and internal spin couplings (spin-spin, spin-orbit and spin-other-orbit). These new spin- and time-dependent many-electron methods will open many new avenues in simulating many-electron dynamics, such as ultrafast charge-transfer, spin transport, spin-crossover, and electronic (and spin) decoherence and dephasing etc. We are also actively working on multi-reference methods in order to understand situations where standard single-reference black-box methods fail, such as near-degeneracies and conical intersections.
First-Principles Nonadiabatic Dynamics
Another active development area in the Li research group focuses on accurately reproducing chemical dynamics for a wide range of system sizes and time scales. To this end, we develop efficient numerical schemes to simultaneously integrate the quantum and classical equations of motion for all or some subset of the nuclear, electronic (spatial and spin), and bath degrees of freedom of chemical systems. With these methods in place, we can probe the dynamical properties of emerging spintronic and molecular plasmonic materials. Specific applications include the propagation and dephasing of non-equilibrium spin states in dilute magnetic semiconductors and plasmonic states induced in noble metal nanowires. Nonradiative relaxation pathways implicated in photochemical reactions are also open for investigation with tractable implementations of ab initio surface hopping methods.
X-Ray Spectroscopy and High-Energy Chemical Physics
X-Ray absorption spectroscopy (XAS) and transient X-Ray absorption (XTA) are complex spectroscopies offering a wealth of information about electronic and nuclear structure. They also remain a challenging problem for theorists. Electronic relaxation due to the core excitations, environmental effects, scalar and spin-orbit relativistic effects, and higher-order multipole coupling to the incident field all need to be accounted for. An ongoing area of research in the Li group is the application of standard single-reference electronic structure methods to these problems through energy specific (ES) approaches. This allows these higher energy excitations to be probed without the need to solve for the lower energy excitations. Real-time TDDFT (RT-TDDFT) can also be applied to determine the effects of electronic relaxation due to the core excitation and non-equilibrium environmental effects.
Chemical Physics of Defects in Nanocrystals
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 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.
G. Donati, D. B. Lingerfelt, C. Aikens, X. Li, “Molecular Vibration Induced Plasmon Decay,” J. Phys. Chem. C, 2017, 121, 15368–15374.
F. Egidi, S. Sun, J. J. Goings, G. Scalmani, M. J. Frisch, X. Li, “Two-Component Noncollinear Time-Dependent Spin Density Functional Theory for Excited State Calculations,” J. Chem. Theory Comput., 2017, 13, 2591–2603.
A. Petrone, D. B. Williams-Young, D. B. Lingerfelt, X. Li, “Ab Initio Excited-State Transient Raman Analysis,” J. Phys. Chem. A, 2017, 121, 3958–3965.
J. J. Goings, F. Egidi, X. Li, “Current Development of Noncollinear Electronic Structure Theory,” Int. J. Quantum Chem., 2017, e25398.
D. B. Lingerfelt, P. J. Lestrange, J. J. Radler, S. E. Brown-Xu, P. Kim, F. N. Castellano, L. X. Chen, X. Li, "Can Excited State Electronic Coherence be Tuned via Molecular Structural Modification? A First-Principles Quantum Electronic Dynamics Study of Pyrazolate-Bridged Pt(II) Dimers," J. Phys. Chem. A, 2017, 121, 1932-1939.
J. J. Goings, D. B. Lingerfelt, X. Li, “Can Quantized Vibrational Effects be Obtained from Ehrenfest Mixed Quantum-Classical Dynamics?,” J. Phys. Chem. Lett., 2016, 7, 5193–5197.