RESEARCH INTERESTS:

Structures, Properties and Dynamics of Clusters and Biopolymers
This work is a combination of analytic theory and computation. Its major topics include a) the phases and phase changes of small systems, b) the topographies of multidimensional potential surfaces and the dynamics these imply, and c) dynamics of proteins and their interactions. The first revealed how first-order phase transitions emerge, in the large-system limit, from the dynamic equilibria of phase-like forms of clusters. The phase behavior of small systems is very different from that of bulk phases. The "phases" of clusters are as much like components as they are phases. Hence they are not restricted to sharp coexistence curves, and the Gibbs Phase Rule is not applicable to them. Studying the small-system analogues of second-order phase transitions revealed two kinds of second-order transitions. The studies of topographies and dynamics have addressed questions such as, "What characteristics of multidimensional potential surfaces will reveal whether a substance is a glass-former or a structure-seeker?" We learn that substances whose potential surface topographies are sawtooth-like are good glass-formers, and substances whose potential surfaces are staircase-like are good structure-seekers. The structure-seekers include materials that crystallize readily, such as alkali halides, and proteins that fold readily into specific kinds of structures. From a knowledge of interlinked sequences of minima and saddles, one can construct kinetic rate coefficients, and from these, a full master equation for any temperature. The master equations can then be used to generate an optimal control procedure to bring an ensemble of clusters or nanoparticles toward a target morphology.

The studies of proteins, in collaboration with colleagues in Chemistry and in Biochemistry and Molecular Biology, have led to new understanding of how the folding process occurs, and how proteins interact and attach to other molecules. One recent finding is how proteins with hydrogen bonds exposed on their surfaces attach to hydrophobic substances, such as lipids. This work is now developing an integrated, multiscale method to model the evolution of protein dynamics from picoseconds to seconds.

Dynamics of Few-Body Systems
Involving both theory and experiment, this research deals with electron correlation in the valence shells of atoms and with simple ionization and collision processes. A fundamental theoretical question here is, "What are the best approximate constants of motion and corresponding quantum numbers for electrons in a many-electron system, independent-particle, as in the Hartree-Fock picture, or collective, and, if collective, of what sort?" In the two-valence-electron atoms, the electrons behave collectively, like very anharmonic rotor-vibrators around their atomic cores. Recent studies have been directed toward predicting and interpreting experiments to reveal the nature and degree of correlation of such electrons. Related experiments and theory involve photoionization and collisions: resonant multiphoton ionization, collisions of monoenergetic, low-energy electrons with excited atoms leaving the electrons accelerated and the atoms de-excited (superelastic collisions), and collisions of excited energy-donor atoms with negative ions, in which the excitation energy of the donors releases the "extra" electron of the negative ion, a process called "Penning detachment."

Finite-time Thermodynamics
Thermodynamics provides limits on the performance of systems that produce or consume work. The limits of classical thermodynamics are based on reversible - and therefore infinitely slow - models. It is possible to develop more realistic limits by incorporating finite-time or nonzero-rate constraints into the definition of a thermodynamic system. Such analyses provoke several questions. First, under what conditions it is possible to define generalizations of thermo-dynamic potentials whose changes give the optimum performance of a finite-time process? Second, what are such functions are and how can we evaluate them? Third, how can we find the paths that yield those optimal performances? Systems studied this way include automobile engines and separation processes such as distillation. There are also more abstract questions: what constitutes the best, minimal set of variables to describe a system well out of equilibrium, for which normal thermo-dynamic variables are not sufficient? How can one describe the simultaneous, coupled flows of mass, heat and entropy in a nonuniform fluid? A problem still more general is the quest for a paradigmatic classification of dissipative systems.

Selected References

Kinetics of model energy landscapes: an approach to complex systems. Phys. Chem. Chem. Phys. 7 (19), 3443-3456 (2005).

Time autocorrelation function analysis of master equation and its application to atomic clusters. J. Chem. Phys.  123, 094103-1-10 (2005).

Properties of a bound ensemble of repelling atoms. Phys. Rev. B 71, 144105-1-8  (2005).

Regularity in Chaotic Transitions on Two-Basin Landscapes. In Geometric Structures of Phase Space in Multi-dimensional Chaos, Adv. Chem. Phys. 130, Part A, 143-170 (2005).

Dielectric modulation of biological water.  Phys. Rev. Lett. 93, 228104-1-4 (2004).

Molecular dimension explored in evolution to promote proteomic complexity. Proc. Natl. Acad. Sci. (USA) 101 (37), 13460-13465 (2004).

The nonconserved wrapping of conserved protein folds reveals a trend towards increasing connectivity in proteomic networks. PNAS, 101(9), 2823-2827 (2004).

Three-photon above-threshold ionization of magnesium. Phys. Rev. A, 68, 063401-1-10 (2003).

Temperature Dependence of Reactions with Multiple Pathways. PCCP, 5, 2589-2594 (2003).

Proteins with H-bond packing defects are highly interactive with lipid bilayers: Implications for amyloidogenesis. Proc. Natl. Acad.Sci., 100, 2391-2396 (2003).

The amazing phases of small systems. Compt. Rend. Phys., 3 1-8 (2002).