Chemical Physics

A pseudorotating state of Li 5 observed in a recent EPR experiment is investigated using the local density functional method. The calculated isotropic spin population indicates that a planar C 2v structure is more consistent with the experimental result than a suggested trigonal bipyramid. A new pseudorotating state based on the planar structure is proposed.

We extend the correspondence between the Renn-Lubensky Twist-Grain-Boundary-A phase in chiral liquid crystals and the Abrikosov mixed state in superconductors to dynamical aspects. We find that for a TGB sample with free boundaries, an external electric field applied along the helical axis induces a uniform translational motion of the grain boundary system - an analogue of the well-known mixed state flux flow. Likewise, an analogue of the mixed state Nernst effect is found. In much the same way in which the flux flow carries intercore electric fields generating Joule heat in an otherwise dissipation-free system, the grain boundary flow carries along polarized charges, resulting in a finite electric conductivity in a ferroelectric.

In this paper we present a method for locating transition states and higher-order saddles on potential energy surfaces using double-ended classical trajectories. We then apply this method to 7- and 8-atom Lennard-Jones clusters, finding one previously unreported transition state for the 7-atom cluster and two for the 8-atom cluster.

The dynamics of spin-boson systems at very low temperatures has been studied using a real-time path-integral simulation technique which combines a stochastic Monte Carlo sampling over the quantum fluctuations with an exact treatment of the quasiclassical degrees of freedoms. To a large degree, this special technique circumvents the dynamical sign problem and allows the dynamics to be studied directly up to long real times in a numerically exact manner. This method has been applied to two important problems: (1) crossover from nonadiabatic to adiabatic behavior in electron transfer reactions, (2) the zero-temperature dynamics in the antiferromagnetic Kondo region 1/2<K<1 where K is Kondo's parameter.

We study the Lyapunov instability of a two-dimensional fluid composed of rigid diatomic molecules, with two interaction sites each, and interacting with a WCA site-site potential. We compute full spectra of Lyapunov exponents for such a molecular system. These exponents characterize the rate at which neighboring trajectories diverge or converge exponentially in phase space. Quam. These exponents characterize the rate at which neighboring trajectories diverge or converge exponentially in phase space. Qualitative different degrees of freedom -- such as rotation and translation -- affect the Lyapunov spectrum differently. We study this phenomenon by systematically varying the molecular shape and the density. We define and evaluate ``rotation numbers'' measuring the time averaged modulus of the angular velocities for vectors connecting perturbed satellite trajectories with an unperturbed reference trajectory in phase space. For reasons of comparison, various time correlation functions for translation and rotation are computed. The relative dynamics of perturbed trajectories is also studied in certain subspaces of the phase space associated with center-of-mass and orientational molecular motion.

Heat capacity curves as functions of temperature for classical atomic clusters bound by pairwise Lennard-Jones potentials were calculated for aggregate sizes from 4 to 24 using Monte Carlo methods. J-walking (or jump-walking) was used to overcome convergence difficulties due to quasi-ergodicity in the solid-liquid transition region. The heat capacity curves were found to differ markedly and nonmonotonically as functions of cluster size. Curves for N = 4, 5 and 8 consisted of a smooth, featureless, monotonic increase throughout the transition region, while curves for N = 7 and 15-17 showed a distinct shoulder in this region; the remaining clusters had distinguishable transition heat capacity peaks. The size and location of these peaks exhibited "magic number" behavior, with the most pronounced peaks occurring for magic number sizes of N = 13, 19 and 23. A comparison of the heat capacities with other cluster properties in the solid-liquid transition region that have been reported in the literature indicates partial support for the view that, for some clusters, the solid-liquid transition region is a coexistence region demarcated by relatively sharp, but separate, melting and freezing temperatures; some discrepancies, however, remain unresolved.

A structural model for intermediate sized silicon clusters is proposed that is able to generate unique structures without any dangling bonds. This structural model consists of bulk-like core of five atoms surrounded by fullerene-like surface. Reconstruction of the ideal fullerene geometry results in the formation of crown atoms surrounded by π -bonded dimer pairs. This model yields unique structures for \Si{33}, \Si{39}, and \Si{45} clusters without any dangling bonds and hence explains why these clusters are least reactive towards chemisorption of ammonia, methanol, ethylene, and water. This model is also consistent with the experimental finding that silicon clusters undergo a transition from prolate to spherical shapes at \Si{27}. Finally, reagent specific chemisorption reactivities observed experimentally is explained based on the electronic structures of the reagents.

The role of shape resonances and many-body effects on universal quantum sticking of ultra cold atoms onto solid surfaces is examined analytically and computationally using an exactly solvable representation of the Dyson equation. We derive the self-energy renormalization of the the transition amplitude between an ultra cold scattering atom and the bound states on the surface in order to elucidate the role of virtual phonon exchanges in the limiting behavior of the sticking probability. We demonstrate that, to first order in the interactions for finite ranged atom-surface potentials, virtual phonons can only rescale the strength of the atom-surface coupling and do not rescale the range of the coupling. Thus, universal sticking behaviour at ultra-low energies is to be expected for all finite ranged potentials. We demonstrate that the onset of the universal sticking behavior depends greatly on the position of the shape resonance of the renormalized potential and for sufficiently low energy shape resonances, deviations from the universal s(E)∝ E − − √ can occur near these energies. We believe that this accounts for many of the low energy sticking trends observed in the scattering of sub-millikelvin H atoms from superfluid 4 He films.

Clusters of sizes ranging from two to five are studied by variational quantum Monte Carlo techniques. The clusters consist of Ar, Ne and hypothetical lighter (`` 1 2 -Ne") atoms. A general form of trial function is developed for which the variational bias is considerably smaller than the statistical error of currently available diffusion Monte Carlo estimates. The trial functions are designed by a careful analysis of long- and short-range behavior as a function of inter-atomic distance; at intermediate distances, on the order of the average nearest neighbor distance, the trial functions are constructed to have considerable variational freedom. A systematic study of the relative importance of n -body contributions to the quality of the optimized trial wave function is made with 2≤n≤5 . Algebraic invariants are employed to deal efficiently with the many-body interactions.

Variations in the band structures of C60-polymers are studied, when pi-conjugation conditions are changed. We look at band structures in order to discuss a metal-insulator transition, using a semi-empirical model with the Su-Schrieffer-Heeger type electron-phonon interactions. We find that electronic structures change among direct-gap insulators and the metal, depending on the degree of pi-conjugations. High pressure experiments could observe such pressure-induced metal-insulator transitions.