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Guohua Tao

 
 
Postdoctoral Associate
Miller Research Group
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
 
  • Quantum effects in condensed phase chemical systems
Quantum effects play a crucial role in many chemical and biochemical progresses in condensed phase, such as electron transfer, proton transfer, coherent electronic energy transfer etc. To describe quantum dynamics, fully quantum-mechanical treatments require formidable computational effort, which grows up exponentially with the degrees of freedom of the system, thus are not feasible for complex molecular systems. On the other hand, classical mechanics cannot describe quantum effects. We still have hope! The semiclassical (SC) initial value representation (IVR) methodology, which has been developed since 1970’s to incorporate quantum effects based on classical trajectories, is one of the most promising tools to fulfill the dream. With great success in a variety of applications, the implementation of SC-IVR methods to complex molecular systems has long been a challenge due to the time-consuming stability matrix analysis and poor convergence related with the sign problem and chaotic trajectories. 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 overcome the above-mentioned difficulty and demonstrate in the first time that the ‘forward-backward’ version of the IVR method (FB-IVR) is capable of capturing detailed quantum coherence in a complex molecular system in full three-dimensional space.  Solvent effects on this vibrational quantum coherence have also been investigated for an I2Arn (n = 1, 6) cluster. A solvent cage consisting of 6 argon atoms reduces the fraction of iodine molecules that dissociate (an example of the ‘cage effect’) and also diminishes, but does not entirely eliminate, quantum coherence in the vibrational motion of the molecules that remain undissociated. The extension of our application of FB-IVR methodology to liquid systems is straightforward.
 
Read more at:
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).
 
  •  The development of semiclassical methodology

    In semiclassical IVR methods, an ensemble of classical trajectories is used to approximate the quantum dynamics of the system. The simplest linearized SC-IVR method only considers 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 contrast, distinct trajectories are included in a forward backward (FB) IVR treatment. 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. The chaotic behavior in the trajectory divergence, which shows up through the FB pre-factor, is also diminished analytically rather than numerically by the FB combination. In FB-IVR, a number of backward trajectories are associated with a single forward trajectory. The idea is that that not every backward trajectory contributes the final result significantly. In order to calculate the contributions of those forward-backward trajectory pairs more efficiently, we develop a Gaussian approximation method, i.e. 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.

    Read more at:
    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. Accepted. (preprint in pdf (1.8M))

  

  • The application of semiclassical IVR to nonadiabatic dynamics

Many photochemical processes involve transitions between different Born-Oppenheimer potential energy surfaces, i.e., nonadiabatic transitions. It is well known that the nuclear coherent motion is an essential component in physiological light-driven (nonadiabatic) processes, and the decay of quantum coherence can determine the nonadiabatic dynamics, therefore accurate description of quantum effects of both electron and nuclei is crucial. However, in prevalent mixed quantum-classical simulations, only a few degrees of freedom are treated quantum mechanically while nuclear dynamics is usually treated classically. Furthermore, a priori knowledge of the quantum decay time is needed in the surface hopping algorithms. The SC-IVR methods are able to treat both electronic and nuclear dynamics on the same dynamical footing; therefore quantum coherent dynamics is inherently incorporated. We are working on the implementation of SC-IVR methods to electronically nonadiabatic chemical dynamics in a variety of simple systems such as a spin-boson system. However, the physics pictures that we could obtain from the study of these simple systems will definitely enrich our understandings of fundamental mechanisms of chemical reaction dynamics and photochemical dynamics, such as energy transfer in photosynthesis systems and proton coupled electron transfer.

 

 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
    
     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. Accepted. (preprint in pdf (1.8M))
 
     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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  • Tao&Miller_JCP.pdf - on Nov 7, 2009 3:45 PM by guohua tao (version 1)
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