Theoretical Chemistry/Chemical Physics

• Quantum Dynamics of Reactions and Clusters
• Theoretical and Computational Methods
• Environmental Chemistry

 

Professor Poirier's research group is concerned with the development and application of new methods for performing accurate quantum dynamics calculations for molecular systems.  These calculations encompass rovibrational spectroscopy (especially highly excited states), reactive scattering of molecules in the gas phase, and resonance phenomena (energies, widths, phase shifts).  Applications of interest include the dynamics of atomic clusters, and thermal rate constants pertinent to environmental chemical kinetics.

The methods development research is motivated by the inadequacy of conventional numerical techniques for dealing with large molecules, insofar as accurate, quantum dynamics calculations are concerned.  Traditionally, the computational effort required scales exponentially with problem size, as a result of which such calculations have to date been limited to systems with four or fewer atoms.  Dr. Poirier's group is exploring a variety of new approaches that improve the computational efficiency by orders of magnitude, thereby making it possible to handle a much larger class of systems than has heretofore been realized.  One such approach relies on a simple classical phase space picture to minimize the number of basis functions needed to represent a given system in a desired energy range.  A parallel approach uses an optimal separable basis approximation to minimize the number of iterations required by numerical linear solvers.  In conjunction with collaborators at Argonne National Laboratories, we are adapting these methods for use on massively parallel computing platforms with hundreds to thousands of nodes. As an alternate means of avoiding the exponential scaling problem, we are also developing quantum trajectory methods, based on the deBroglie-Bohm formulation of quantum mechanics.

The cluster research seeks to understand the various "phase changes" observed in nanoscale structures.  These can be regarded as prototypes of phase transitions in bulk matter, yet can occur with as few as six or seven atoms.  The group has an interest in solid-liquid phase changes, as well as those associated with solvation. In all cases, dynamics plays a crucial role.  Quantitative analysis of the solid-liquid phase change, for instance, requires accurate knowledge of both the low vibrational states, and the highly excited states in the metastable band.  From this data, fundamental questions can be answered (e.g., how do latent heats depend on cluster size?).  Specific systems include clusters of rare gas atoms (He, Ne, Ar), as well as certain transition metal clusters (Ag, Au).

The chemical reactions that occur in combustion, and in the atmosphere, are of great practical interest. The broad goal of environmental chemistry is to understand, predict, and control how noxious compounds are generated, and how they affect the atmosphere and other environmental systems.  This can be accomplished with good theoretical models; however, the utility of such models is limited by a lack of accurate rate constants for the constituent elementary chemical reactions.  The specific goal of the environmental chemistry research, therefore, is to perform accurate reaction rate calculations for four-to-six atom reactions that play a key role in environmental systems, e.g.

Cl + O₃ → ClO + O₂

H + CH₄ → H₂ + CH₃

   

Selected Publications "Reconciling Semiclassical and Bohmian Mechanics: I Stationary States", Poirier, B. J. Chem. Phys. 2004, 121, 4501. "Accurate and Highly Efficient Calculation of the Highly Excited Pure OH Stretching Resonances of O(1D)HCl, Using a Combination of Methods", Bian, W.; Poirier, B. J. Chem. Phys. 2004, 121, 4467. "Quantum Dynamics Calculations Using Symmetrized Orthogonal Weyl-Heisenberg Wavelets with a Phase Space Truncation Scheme. III. Representations and Calculations", Poirier, B.; Salam, A. J. Chem. Phys. 2004, 121, 1704. "A Quantum Dynamical Study of the He+ + 2He → He2+ + He Reaction." Xie, J.; Poirier, B.; Gellene, G. J. Chem. Phys. 2003, 119, 10678. "Eigenspectra Calculations Using Cartesian Coordinates and a Rotational Symmetry Adapted Lanczos Method", Montgomery, J.; Poirier, B. J. Chem. Phys. 2003, 119, 6609. "A Preconditioned Inexact Spectral Transform Method for Calculating Resonance Energies and Widths, as Applied to HCO",  Poirier, B.; Carrington, Jr., T. J. Chem. Phys. 2002, 116, 1215.  "Phase Space Optimization of Quantum Representations: Three-body systems, and the bound states of HCO", Poirier, B.;  Light, J. C. J. Chem. Phys. 2001, 114, 6562.