Haobin Wang of New Mexico State University is supported by an award from the Theory, Models and Computational Methods program to develop unified multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) theory to simulate quantum dynamics as well as equilibrium properties for complex systems that contain both distinguishable and indistinguishable particles. The key to this method development is to employ the second quantization formalism in the actual variational calculation and to express the wave function by a recursive, layered expansion in this representation. A primary focus of the research projected involves the study of correlated electronic-nuclear dynamics in electron transport, in particular
(1) The ultrafast electron transfer at dye-semiconductor interfaces (Graetzel solar cells) and (2) Electron transport through molecular junction devices.
Electron transport plays an important role in many technological processes including energy conversion in solar cells and molecular electronic devices. A diverse group of researchers contribute to this project including undergraduates from under-represented groups, community college students, graduate students and collaborators from leading research groups in the U.S. and Europe.
During the period of 2010-2014, this NSF grant supported a variety of research and educational efforts in the PI's research group at New Mexico State University. The objective of the sponsored research is to develop a rigorous and efficient variational method, the multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) theory, to study correlated quantum dynamics of ultrafast electron transfer processes, and to apply the theory to the electron injection process at dye-semiconductor interfaces (the primary step in Gratzel solar cells) and the electron transport through molecular junction devices. The research accomplishments are briefly summarized as follows. To the first end, significant progress has been made in developing a unified ML-MCTDH theory to simulate quantum dynamics for complex systems that contain both distinguishable and indistinguishable particles. The ML-MCTDH in second quantization representation (SQR), has been integrated into the original formulation to treat systems of identical fermions, bosons, and combinations of both distinguishable and indistinguishable particles. The ensemble average over many independently-propagated wave functions were obtained from a Monte Carlo importance sampling procedure, in which both real- and imaginary-time propagations were handled by the ML-MCTDH-SQR theory. Alongside the formal development of the theory, the computer program, developed solely by the PI to simulate quantum dynamics for a broad class of physical problems, has been updated and enhanced significantly. It has been implemented on massively parallel computer platforms and is currently used by several groups in the world. The development and maintenance of this computer program represent a significant portion of the PI's research effort. With respect to the application of the theory, the ML-MCTDH method has been used to study correlated dynamics in nonequilibrium quantum transport through single-molecule junctions, and to simulate ultrafast electron transfer dynamics in condensed phase systems and nanosystems. In particular, it provided numerically exact results for systems with strong electronic and vibrational couplings. The results demonstrate the complex interplay of electronic and vibrational dynamics. For example, strong electronic-vibrational coupling may result in a pronounced suppression of the electrical current, which is accompanied by the formation of a phonon blockade. Such a correlated dynamics is often missed by the current implementation of the nonequilibrium Green's function type of methods. Our numerically exact study provides reliable benchmark results that can be used to gain fundamental understanding of the underlying physical processes and to guide the development of approximate theories. Overall, more than twenty peer-reviewed papers have been published under the support of this grant. With respect to the broader impact of our work, the grant has fostered collaborative research within and outside the PI's institution. These collaborations resulted in both scientific publications and students training. Research outcomes have been presented at seminars, conferences and summer schools. The numerical algorithms developed in this project starts to have impact on other scientific disciplines such as applied mathematics and engineering, and thus goes beyond theoretical chemistry. The grant was used to support the research of one graduate student and ten undergraduate students. Several of them are from either economically disadvantaged or historically underrepresented groups, and have found the research experience indispensable for their college training and future career. Furthermore, the PI and his research group have carried out extensive outreach activities to expose the high school students of computational research in chemistry.
