Research

In the Levine group we work on a variety of problems, ranging from applied chemistry and physics to method development to high performance computing.  Below are brief discussions of several ongoing research projects in our group.

Material Photochemistry and Photophysics
trapping-300x217Optoelectronic devices, e.g. photovoltaics, photocatalysts, and light emitting diodes, operate by converting the energy of excited electrons into useful work.  The efficiency of such devices depends on a complex series of physical processes, e.g. absorption, charge separation, localization (trapping) of charge carriers, and radiative and non-radiative decay.  Empirical optimization of optoelectronic devices has led to impressive increases in efficiency in recent years, but in many cases an atom-by-atom, electron-by-electron understanding of the underlying fundamental processes is lacking, thus limiting our ability to rationally design materials for specific applications.  It is the goal of our group to develop and apply computational methods in order to build such understanding.  Specifically, our group applies electronic structure theory and nonadiabatic ab initio molecular dynamics simulations to model the electronic and nuclear motions that follow the excitation of semiconductor clusters and nanoparticles, with specific attention to electronically excited defects and their role in facilitating recombination.  As described below, method and software developed in our group allows us to model recombination with unprecedented accuracy and to expand the length- and time-scales accessible to us via simulation.

Representative publications:

Understanding Nonradiative Recombination through Defect-Induced Conical Intersections, Y. Shu, B. S. Fales, W.-T. Peng, and B. G. Levine, J. Phys. Chem. Lett., 8, 4091 (2017)

First Principles Study of Non-Radiative Recombination in Silicon Nanocrystals: The Role of Surface Silanol, Y. Shu and B. G. Levine, J. Phys. Chem. C, 120, 23246 (2016)

Surface Structure and Silicon Nanocrystal Photoluminescence: The Role of Hypervalent Silyl Groups, Y. Shu, U. R. Kortshagen, B. G. Levine, and R. J. Anthony, J. Phys. Chem. C, 119, 26683 (2015)

Defect-Induced Conical Intersections Promote Nonradiative Recombination, Y. Shu, B. S. Fales, and B. G. Levine, Nano Lett., 15, 6247 (2015)

Nonradiative recombination via conical intersections arising at defects on the oxidized silicon surface.  Y. Shu and B. G. Levine, J. Phys. Chem. C, 119, 1737 (2015)

Do excited silicon-oxygen double bonds emit light?  Y. Shu, B. G. Levine, J. Phys. Chem. C, 118, 7669 (2014)

Polaronic relaxation by three-electron bond formation in graphitic carbon nitrides.  G. A. Meek, A. D. Baczewski, D. J. Little, B. G. Levine, J. Phys. Chem. C, 118, 4023 (2014)

Non-radiative recombination via conical intersection at a semiconductor defect.  Y. Shu, B. G. Levine, J. Chem. Phys., 139, 081102 (2013)

High-Energy Nonadiabatic Dynamics

A number of fascinating and important chemical reactions involve nonadiabatic molecular dynamics at energies above the ionization threshold (e.g. atmospheric reactions and molecules in strong laser fields).  These reactions pose unique theoretical and computational challenges due to the sheer number of electronic states involved and the diversity of their characters.  We are developing novel methods capable of modeling coupled electron-nuclear dynamics in this manifold of high-energy states, and applying them to develop an understanding of chemistry above the ionization threshold.

Representative publications:

Mechanisms and time-resolved dynamics for trihydrogen cation (H3+) formation from organic molecules in strong laser fields, N. Ekanayake, M. Nairat, B. Kaderiya, P. Feizollah, B. Jochim, T. Severt, B. Berry, K. R. Pandiri, K. D. Carnes, S. Pathak, D. Rolles, A. Rudenko, I. Ben-Itzhak, C. A. Mancuso, B. S. Fales, J. E. Jackson, B. G. Levine, and M. Dantus, Sci. Rep., 7, 4703 (2017)

Polyatomic molecules under intense femtosecond laser irradiation.  A. Konar, Y. Shu, V. V. Lozovoy, J. E. Jackson, B. G. Levine, and M. Dantus, J. Phys. Chem. A, 118, 11433 (2014) (Feature Article) Cover

Multireference Method Development

Non-radiative recombination, the process by which the energy of excited electrons is lost as heat, limits the efficiency of certain optoelectronic devices (e.g. photovoltaics, light emitting diodes, photocatalysts), while determining the functionality of others (e.g. fluorescence-based sensors).  Such recombination is analogous to non-radiative decay in molecules, which necessarily involves complex multideterminantal electronic wavefunctions.  Our research has shown that similarly complex wavefunctions are involved in the photochemistry of semiconductor nanomaterials.  A very widely used method for the description of such wavefunctions is the complete active space self-consistent field (CASSCF) approach, which offers a desirable balance between computational cost and accuracy.  However, the complexity of the CASSCF equations leads to some undesirable features, including convergence difficulties, multi-valued potential energy surfaces, a propensity for unphysical wavefunction symmetry breaking, and results which depend strongly and unpredictably on user-defined parameters.  Our group is developing simpler alternatives to CASSCF which address these issues.  The goal is to develop multireference methods which maintain CASSCF’s ability to describe complex multideterminantal wavefunctions while eliminating its less desirable qualities.  At the same time, the simpler equations solved in our approaches result in a lower computational cost than CASSCF, thus allowing an extension of the length- and time-scales of our simulations.

Representative publications:

Complete active space configuration interaction from state-averaged configuration interaction singles natural orbitals: Analytic first derivatives and derivative coupling vectors, B. S. Fales, Y. Shu, B. G. Levine, and E. G. Hohenstein, J. Chem. Phys., 147, 094104 (2017)

Robust and Efficient Spin Purification for Determinantal Configuration Interaction, B. S. Fales, E. G. Hohenstein, and B. G. Levine, J. Chem. Theory Comput., 13, 4162 (2017)

Configuration interaction singles natural orbitals: an orbital basis for an efficient and size intensive multireference description of electronic excited states, Y. Shu, E. G. Hohenstein, and B. G. Levine, J. Chem. Phys., 142, 024102 (2015)

Reducing the propensity for unphysical wavefunction symmetry breaking in multireference calculations of the excited states of semiconductor clusters.  Y. Shu, B. G. Levine, J. Chem. Phys., 139, 074102 (2013)

Nanoscale Multireference Quantum Chemistry: Full Configuration Interaction on Graphical Processing Units, B. S. Fales and B. G. Levine, J. Chem. Theory Comput., 11, 4708 (2015)

Graphics Processing Unit Computing

Graphics processing units (GPUs) are low-cost, high-performance computer processors designed for graphical applications (i.e. video games).  Fortunately for scientists, both graphical rendering and quantum chemistry depend on the ability of the processor to perform fast floating point linear algebra operations.  By taking advantage of this similarity, we are able to apply GPUs to accelerate quantum chemical calculations, with performance ranging from tens to hundreds of time faster than a traditional CPU core.  By applying GPU processors in conjunction with the methodological advances described above, we are working towards extending multireference quantum chemical simulations from the molecular- to the nano-scale.

Nanoscale Multireference Quantum Chemistry: Full Configuration Interaction on Graphical Processing Units, B. S. Fales and B. G. Levine, J. Chem. Theory Comput., 11, 4708 (2015)

Robust and Efficient Spin Purification for Determinantal Configuration Interaction, B. S. Fales, E. G. Hohenstein, and B. G. Levine, J. Chem. Theory Comput., 13, 4162 (2017)

Defect-Induced Conical Intersections Promote Nonradiative Recombination, Y. Shu, B. S. Fales, and B. G. Levine, Nano Lett., 15, 6247 (2015)

Simulated evolution of fluorophores for light emitting diodes, Y. Shu and B. G. Levine, J. Chem. Phys., 142, 104104 (2015)

Nonadiabatic Molecular Dynamics Development

The modeling of the dynamics of molecules and materials after excitation requires the treatment of both nuclei and electrons as quantum mechanical particles.  The coupling between nuclear and electronic motion (nonadiabatic coupling) facilitates non-radiative processes which limit the efficiency of many optoelectronic devices.  The complexity of such simulations increases dramatically when the system to be studied is a large, nanoscale system, which necessarily has a larger number of more weakly coupled electronic states.  Within the framework of the ab initio multiple spawning method, we work to improve the accuracy of our description of non-adiabatic dynamics, with a focus on the challenges associated nanomaterials.

Representative publications:

The Best of Both Reps: Diabatized Gaussians on Adiabatic Surfaces, G. A. Meek and B. G. Levine, J. Chem. Phys., 145, 184103 (2016)

Wave Function Continuity and the Diagonal Born-Oppenheimer Correction at Conical Intersections, G. A. Meek and B. G. Levine, J. Chem. Phys., 144, 184109 (2016)

Accurate and efficient evaluation of transition probabilities at unavoided crossings in ab initio multiple spawning, G. A. Meek and B. G. Levine, Chem. Phys., 460, 117 (2015)

Evaluation of the time-derivative coupling for accurate electronic state transition probabilities from numerical simulations.  G. A. Meek and B. G. Levine, J. Phys. Chem. Lett., 5, 2351 (2014)

Evolutionary Design of Optoelectronic Materials

A tremendous amount of time and effort is spent experimentally screening molecules for various optoelectronic applications, but in recent years computational chemistry methods and high performance computers have developed to the point that it is possible to approximate the photophysical properties of a large molecule in a matter of minutes.  Taking advantage of these developments, we use genetic algorithms in conjunction with electron structure methods to computationally evolve molecules with a desirable set of properties.  Multi-GPU parallelism and an intelligent choice of the “primordial soup” from which molecules are evolved allows us to search a massive molecular space before a single wet experiment is run.  We hope this strategy will accelerate the development of materials for energy conversion, light emission, and other applications.

Representative publications:

Simulated evolution of fluorophores for light emitting diodes, Y. Shu and B. G. Levine, J. Chem. Phys., 142, 104104 (2015)

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