Postdoctoral Associate
Room 17, Gilman Hall
Pitzer Center for Theoretical Chemistry
Department of Chemistry
University of California at Berkeley
Berkeley, CA 94720
Phone: (510) 642-1463
Email: gtao at berkeley.edu
Research Interests
Our research interests span a wide range of areas with the same focus on chemical reaction dynamics. We develop new theoretical methods and ideas on effectively describing quantum effects in chemical dynamics. Our efforts will help elucidate the underlying mechanisms of chemical or biochemical reactions and how to “improve” them by changing reaction rates, branching ratios and by making artificial alternatives.
Some research projects explain our work in details as the follows.
- Electronically nonadiabatic chemical reaction dynamics
The famous Bohn-Oppenheimer (BO) approximation, which assumes that nuclei evolve along a single potential energy surface that is defined by fast-moving electrons, is the milestone in understanding chemical reactions. However, in many photochemical processes, such as photodissociation reactions, photoisomerization reactions, or chemical dynamics in photosynthesis, BO approximation breaks down and nonadiabatic transitions take place between different BO potential energy surfaces. Nonadiabatic effects here play a crucial role in determination of important features in chemical reactions, such as reaction rates, and branching ratios. The challenge is to describe the coupling between electronic and nuclear motions accurately. The ideal theoretical method for treating electronically nonadiabatic chemical dynamics should be able to treat both electronic and nuclear dynamics on the same dynamical footing. Furthermore quantum coherent dynamics should be properly described since it is the key to understand many important phenomena, such as energy transfer in photosynthesis. The initial value representation (IVR) methods of semiclassical (SC) theory, in combination with the Meyer-Miller-Stock-Thoss (MMST) theory, are very suitable for use in this sense. We have successfully applied our methods to a model photosynthetic system to describe the coherent energy transfer dynamics in photosynthesis. We are currently working on the efficient implementation of these SC methods to electronically nonadiabatic chemical dynamics in a variety of complex molecular systems.
Reference:
- Quantum dynamics in condensed phase chemical systems
Many important chemical or biochemical processes take place
in condensed phase. People may think of that the system-bath interactions can
wash out any quantum coherence in condensed phase dynamics. However in a
nonequilibrium system, quantum coherence may play a very important role. For
example, the energy transfer in the light harvesting protein-pigment complex of
photosynthesis is actually coherent! Quantum coherence and other quantum
effects, such as tunneling effects are responsible for many very interesting
observations in chemical dynamics in condensed phase. We are working on
applying semiclassical theory and statistical mechanics to study nonequilibrium
(and equilibrium) quantum dynamics in condensed phase chemical systems, such as
hydrogen or proton transfer in chemical reactions, and coherent energy transfer
in photosynthesis.
In semiclassical IVR methods, an ensemble of classical trajectories is used to approximate the quantum dynamics of the system. Quantum effects are included by considering the interferences between different trajectories, which requires extra efforts than classical molecular dynamics simulations. The simplest linearized SC-IVR method only considers those trajectories close infinitesimally to each other, thus cannot describe true quantum coherence, although it does simplify the calculations of the full SC-IVR integrals drastically. In cases of that more accurate descriptions are needed, higher level SC-IVR methods, such as the forward-backward (FB) IVR, or even the full SC-IVR method are required. The main challenges in these higher level SC-IVR calculations are the well-known sign problem, which comes from the phase of the integrand (the difference of the action integrals for two different trajectories) and the chaotic behavior in the trajectory divergence. So the problem is that how we can make the implementation of SC-IVR efficient enough to treat complex molecular systems in general.
* FB-IVR
The construction of forward-backward trajectory pairs provides an efficient re-summation scheme for the double phase space averages in full SC-IVR integrals, in which extraneous oscillatory parts of the phase space averages that are involved are eliminated analytically rather than numerically by the FB combination. In the case study of the time-dependent probability distribution of the I2 vibrational coordinate following photo-excitation of I2 in a rare gas cluster, we demonstrated in the first time that FB-IVR is capable of capturing detailed quantum coherence in a complex molecular system in the full three-dimensional space. Solvent effects on this vibrational quantum coherence have also been investigated for an I2Arn (n <= 30) cluster. The extension of our application of FB-IVR methodology to liquid systems is straightforward.
We further developed a Gaussian approximation method in order to calculate the contributions of forward-backward trajectory pairs more efficiently, by assuming the contributions distribute in a form of a few narrow Gaussian regions in the ps space (here ps represents a momentum jump connecting the forward and the backward trajectory). Not surprisingly, our method shows that the most significant contribution to the quantum coherence effects comes from distinct trajectory pairs, i.e. large value of ps. More importantly it provides some intuitive insights for making proper and efficient semiclassical approximations in describing quantum coherence.
* Full SC-IVR
Developed a new efficient time-dependent importance sampling method to perform full SC-IVR calculations for correlation functions and applied it to study vibrational quantum coherence, and the benchmark reaction dynamics of H+H2.
EDUCATION
l Ph.D., Theoretical Chemistry, Brown University, Providence, RI, May 2007
Thesis Title: “Molecular Dynamics Simulation and Theoretical Analysis of Ultrafast Spectroscopy and Rotational Intermolecular Dynamics in Liquids”
Advisor: Professor Richard M. Stratt
l M.S., Applied Mathematics, Brown University, Providence, RI, May 2004
l B.S., Chemistry, Peking University, Beijing, China, July 2000
PUBLICATIONS
9. Guohua Tao and William H. Miller, "Time-dependent importance sampling in semiclassical initial value representation calculations for time correlation functions", J. Chem. Phys. 135, 024104 (2011).
8. Joerg Tatchen, Eli Pollak, Guohua Tao, and William Miller, "Renormalization of the frozen Gaussian approximation to the quantum propagator", J. Chem. Phys. 134, 134104 (2011).
6. Guohua Tao and William H. Miller, "Gaussian Approximation for the Structure Function in Semiclassical Forward-Backward Initial Value Representations of Time Correlation Functions", J. Chem. Phys. 131, 224107 (2009).
5. Guohua Tao and William H. Miller, "Semiclassical description of vibrational quantum coherence in a three dimensional I2Arn(n<=6) cluster: A forward-backward initial value representation implementation", J. Chem. Phys. 130, 184108 (2009).
4. Guohua Tao and Richard M. Stratt, “Anomalously Slow Solvent Structural Relaxation Accompanying High-Energy Rotational Relaxation”, (James T. (Casey) Hynes Festschrift), J. Phys. Chem. B, 112, 369 (2008).
3. Guohua Tao and Richard M. Stratt, “The Molecular Origins of Nonlinear Response in Solute Energy Relaxation: The Example of High-energy Rotational Relaxation”, J. Chem. Phys. 125, 114501 (2006).
2. Amy C. Moskun, Askat E. Jailaubekov, Stephen E. Bradforth, Guohua Tao and Richard M. Stratt, “Rotational Coherence and a Sudden Breakdown in Linear Response Seen in Room-Temperature Liquids”, Science, 311, 1907 (2006).
1. Guohua Tao and Richard M. Stratt, “Why Does the Intermolecular Dynamics of Liquid Biphenyl so Closely Resemble that of Liquid Benzene? Molecular Dynamics Simulation of the Optical-Kerr-Effect Spectra”, J. Phys. Chem. B, 110, 976 (2006).
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