Research:

Theoretical and Computational Chemistry

Research Program Summary

Our research group is carrying out theoretical and computational studies of chemical and biochemical reaction dynamics, potential energy surfaces, and molecular solvation. The tools are quantum mechanics, classical and semiclassical mechanics, and molecular modeling. Additional work focuses on electronic structure theory, molecular energy levels, Feynman path integrals, and thermochemistry. Application areas include nanoscale science, quantum photochemistry, combustion, biochemistry (enzymes and neurochemistry), catalysis, high-energy species, and atmospheric and environmental chemistry.

Quantum mechanical dynamics

A critical focus area in our research group is quantum mechanical dynamics. We have developed an efficient linear algebraic variational method for calculating converged quantum mechanical transition probabilities for reactive collisions. At present, the main application area is quantum photochemistry, i.e., the utilization of electronic excitation energy to promote chemical reactions.

Other current work based on converged quantal dynamics calculations of chemical reactions includes the calculation of reaction rate coefficients as a function of temperature, studies of the utilization and disposal of vibrational and rotational energy in chemical reactions, and the characterization of quantized dynamical bottle necks for reactions (transition state spectroscopy).

An exciting area of special interest is the application of these methods to enzyme reactions, and we have made good progress at incorporating quantum mechanical effects such as zero point energy, tunneling, and substrate polarization into the quantitative modeling of proton and hydride transfer reactions catalyzed by enzymes. An interesting area for future study is the incorporation of quantal effects in enzyme reactions that proceed by radical mechanisms.

The development of new methods for studying photochemistry and other non-Born-Oppenheimer processes is another active field of research. The convenient generation of potential energy surfaces in the dynamically preferred electronic representation and the incorporation of decoherence into non-Born-Oppenheimer trajectories are two areas of the utmost importance where progress has been made.

In related work we are developing improved Monte Carlo methods for the calculation of Feynman path integrals for quantum statistical mechanics.

Semiclassical dynamics: VTST and multi-surface trajectories

Electronically adiabatic reactions. Electronically adiabatic reactions are those that take place entirely in the ground electronic state, i.e., thermally activated reactions on a single potential energy surface. We have developed a powerful technique for studying such reactions, namely, variational transition-state theory with multidimensional semiclassical tunneling contributions (VTST). VTST involves finding the free energy bottleneck for over barrier processes and the optimal tunneling paths for through-barrier processes. This theory has been developed for reactions in the gas phase, in liquid solution, on metallic surfaces, and in enzyme active sites. We are studying the role of tunneling and quantum mechanical vibrational energy on rate constants, kinetic isotope effects, and state-selective chemistry. We may include both equilibrium and nonequilibrium solvation effects for reactions in the condensed phase. Application areas include combustion, atmospheric chemistry, environmental chemistry, clusters (from microhydrated species to nanoparticles), and catalysis (heterogeneous, organometallic, and biological).

Electronically nonadiabatic collisions. We are studying semiclassical trajectory methods for reactive collisions involving coupled potential energy surfaces. We are testing existing methods against accurate quantum mechanical results, and we are developing new methods. Two types of semiclassical (or rather mixed quantum-classical) methods are under study: trajectory surface hopping (also called molecular dynamics with quantum transitions) and self-consistent potential methods (also called time-dependent self-consistent-field methods). We have recently developed a new method that combines the best features of both of these approaches into a single formalism. This new method is called decay of mixing with coherent switches, and it is more accurate than previously available methods for the whole range of problems encountered in photochemistry. Furthermore, it is practical to apply this method, at least when zero-point energy (ZPE) effects are not expected to be too large, to both simple and complex photochemical reactions, and we are currently carrying out such calculations for ammonia, OH...HH, bromoacetyl chloride, and Na...HF. When ZPE effects play a major role, traditional mixed quantum-classical methods may need to be improved and we are currently investigating the effect of these adjustments on simulation results.

Force fields, potential energy surfaces, direct dynamics, and computational thermochemistry

In addition to dynamics calculations, a large effort is being devoted to the development of new potential energy functions and force fields, using new techniques of ab initio and semiempirical electronic structure theory, as well as molecular modeling techniques. Knowledge of the potential energy surface or force field is a prerequisite for either dynamics or thermochemistry calculations.

One area of active work is the extension of molecular mechanics force fields to be able to treat reactive systems that involve bond breaking. An approach called multi-configuration molecular mechanics (MCMM) has been developed for this purpose, and it is very promising.

One area of special concentration is in the interface of electronic structure theory and dynamics. We are developing a variety of single-level and dual-level methods for direct dynamics calculations, where direct dynamics denotes the calculation of rate constants or other dynamical quantities directly from electronic structure calculations without the intermediacy of fitting a potential energy function. In such a case the potential energy surface is implicit but is never actually constructed.

A very exciting recent development is the parameterization of multi-coefficient methods for scaling components of the correlation energy and extrapolating electronic structure calculations to an infinite basis set. These methods allow one to calculate accurate gas-phase heats of formation, atomization energies, and potential energy surfaces for large systems at affordable cost. These methods have better scaling properties than pure ab initio calculations, and they often yield more accurate results with far less computer time. We have now shown how these methods can be improved by adding static correlation with density function theory for even great performance-to-cost ratios.

The direct calculation of free energies from potential energy surfaces, without first calculating the energy spectrum, is also of great interest, and we are developing improved Monte Carlo sampling methods for doing this by the Feynman path integral method.

Solvation effects

Solvation effects are important for several of the above mentioned projects, and our group has a special focus on solvation effects; much of this work involves a close collaboration with Professor ChrisCramer and his research group.

The general goal of our work on solvation is to allow the treatment of energetics and dynamics in the condensed phase to be as accurate as their treatment for gas-phase species and processes. To this end, we are working on theoretical problems involving solvation effects on organic, biochemical, and environmental processes in aqueous and nonaqueous solutions. The role of the solvent in polarizing the solute is especially interesting. Solvation models for both aqueous and organic solvents have been developed and are under development, and a variety of applications to structure and reactivity in solution are underway.

One important aspect of our solvation work is the incorporation of the new solvation models into dynamics calculations, including both equilibrium and nonequilibrium solvation, and spectroscopy. The dynamics calculations employ VTST methods discussed above, and spectroscopic applications involve a new vertical excitation model. We are also applying solvation models to nonhomogeneous media (e.g., cell membrane boundaries), to supercritical fluids, and to environmentally green ionic liquids.

Further information about solvation models and solvation software is available on another page: http://comp.chem.umn.edu/solvation. See also the MURI home page and the Computational Neuroscience home page.

Biochemistry

Many enzymatic reactions involve proton and hydride transfer, but until recently techniques for simulating the dynamics of these processes were usually based entirely in classical mechanics. We are working on a number of initiatives for including quantum effects in biological simulations. This includes tunneling, zero point effects, and the effect of quantization on thermally averaged quantities. We have shown that proton transfers catalyzed by enolase and hydride transfer catalyzed by liver alcohol dehydrogenase are dominatedby quantum mechanical events, and that these can be well modeled by semiclassical dynamics methods developed in our group. Further development are applications are underway.

An important application of solvation modeling is the calculation of the partitioning of organic and biological molecules between aqueous and cell membranes. This has an important effect on bioavailability of drugs. We are developing and applying new molecular modeling methods that make such calculations more accurate.

Much of our work on enzyme kinetics involves collaboration with Professor Jiali Gao.

Nanomaterials

Nanotechnology is the art of manipulating materials on a scale of the order of a nanometer, to build molecular scale devices or to take advantage of the unique chemical, physical, and material properties of nanostructured materials. Our research in this area focuses on computational studies of nanoparticle growth and dynamics. We are concerned with the development and implementation of new methods for modeling and simulation of nanoparticles and their elementary processes, including nucleation, deposition, melting, and surface reactions. Nanoscale systems present a challenge to computation because they display properties that are not well modeled by methods developed for use in bulk simulations and because they are expensive to treat using methods developed for molecular systems. We have therefore taken a bootstrap approach with the goal of developing a set of accurate methods for predicting the energetics and structures of Al particles from Al dimer to the bulk, including nanoparticles with 40 to 200 Al atoms. Critical to the success of this project is the development of new electronic structure methods for aluminum nanoparticles. To study larger aluminum nanoparticles, we are currently also developing a novel tight binding-configuration interaction (TBCI) method that incorporates charges non-iteratively into tight binding by applying a configuration interaction-type treatment to the tight binding wavefunction. We are especially concerned with multi-scale modeling, i.e., the development of new techniques for extending the time and length scales of simulations and their application to problems involving semiconductor nanoparticles and metal nanoparticles. To study of the importance of quantum effects in nanoparticle reactivity, for example, the reaction of metal particles with hydrocarbons and hydrocarbon fragments, we are developing multilevel methods, such as QM/MM methods, that combine quantum mechanics (QM) and molecular mechanics (MM). The efficiency of these methods potentially allows one to perform accurate calculations for large reactive systems over long time scales. For the simulation of systems with non-localized active areas, it is necessary to adaptively redefine the region to be treated by quantum mechanics. For such systems, we are developing new methods for combining multilevel methods with modern sampling schemes, such as our molecular dynamics code, ANT, or Monte Carlo codes. This work is carried out as part of the research program of the Center for NanoEnergetics Research.

Integrated Tools for Computational Chemical Dynamics

This is a joint project between the Chemistry Department of the University of Minnesota and the Supercomputing Institute of the University of Minnesota (PI: D. G. Truhlar) and Pacific Northwest National Laboratory (co-PI: Bruce C. Garrett). The University of Minnesota faculty investigators participaating in this initiative, in addition to DGT, are: Chris Cramer, Jiali Gao, Ilja Siepmann, and Darrin York. The goal of this project is to develop more powerful simulation methods and incorporate them into a user-friendly high-throughput integrated software suite for chemical dynamics. Recent advances in computer power and algorithms have made possible accurate calculations of many chemical properties for both equilibria and kinetics. Nonetheless, applications to complex chemical systems, such as reactive processes in the condensed phase, remain problematic due to the lack of a seamless integration of computational methods that allow modern quantum electronic structure calculations to be performed with state-of-the-art methods for electronic structure, chemical thermodynamics, and reactive dynamics. These problems are often exacerbated by invalidated methods, non-modular and non-portable computer codes, and inadequate documentation that drastically limit software reliability, throughput, and ease of use. The goal of the Integrtated Tools consortium is to develop an integrated software suite that combines electronic structure packages with dynamics codes and efficient sampling algorithms for the following kinds of condensed-phase modeling problems:

thermochemical kinetics and rate constants
photochemistry and spectroscopy
chemical and phase equilibria
computational electrochemistry
heterogeneous catalysis

Photochemical creation of excited states offers a means to control chemical transformations because different wavelengths of light can be used to create different vibronic states, thereby directing chemical reactions along different pathways. It is crucial to understand how energy deposited into the system is used; this is particularly complicated in condensed phase systems where many channels lead to dissipation of excess energy. Similar opportunities and challenges present themselves in the areas of electrochemistry and catalysis. This work is supported in part by the Office of Naval Research. For further information, please see the Integrated Tools home page and Grand Challenge Project: Computational Chemical Dynamics of Complex Systems.

Grand Challenge: Computational Chemical Dynamics of Complex Systems

"Computational Chemical Dynamics of Complex Systems" is a Computational Grand Challenge project of The William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a U.S. Department of Energy national scientific user facility located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. Resources are provided by EMSL's Molecular Science Computing Facility (MSCF). The project is a collaborative effort involving scientists in the Department of Chemistry at the University of Minnesota and scientists at PNNL. Our consortium is focusing on a variety of condensed-phase modeling problems including thermochemical kinetics and rate constants, photochemistry and spectroscopy, chemical and phase equilibria, electrochemistry, and heterogeneous catalysis.

Earth and Planetary Materials Research

Research is being carried out on the incorporation of quantum mechanics into the simulations of materials, especially under conditions relevant to earth and planetary sciences. This includes work on density functional theory and quantum dynamics. For further details see the VLab home page.

Some of the material on this Web page is based in part upon work supported by the National Science Foundation under grant No. CHE-0349122, by the U.S. Department of Energy, Office of Basic Energy Sciences under grant No. DE-FG02-86ER13579, by the Defense-University Research Initiative in Nanotechnology (DURINT) and the Multidisciplinary University Research Initiative (MURI) managed by the Army Research Office, and by the Minnesota Partnership for Biotechnology and Medical Genomics. Opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation, U. S. Department of Energy, or other sponsors.

This document last modified Thursday, 24-Jul-2008 13:38:09 CDT