Volume 111, Issue 3, 15 July 1999
 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


A comment on dielectric hole burning
View Description Hide DescriptionThe method of dielectrichole burning is used to distinguish a homogeneous broadened spectrum from a distribution of relaxation times in a dielectric material. After stimulation with a sinusoidal high electric field a timedependent shift of the response function is observed. This shift is considered to be induced by a frequency selective absorption of energy and an increase of a local fictive temperature. A computer simulation, which is based upon the model of asymmetric doublewell potentials for dipole orientations, also shows a timedependent shift of the response function after applying and removing a sinusoidal high electric field. This behavior is explained considering the timedependent polarization only. It is shown in this article that it is not necessary to assume an increase of a local fictive temperature caused by a frequency selective absorption of energy.

A simple model of tracerdiffusion of nonspherical Brownian particles
View Description Hide DescriptionWe present a Brownian dynamic simulation of the translational and rotational motion of an interacting nonspherical Brownian particle. This simulation experiment involves an idealized model system of a suspension of spherical colloidal particles with which the nonspherical particle interacts. The latter is represented as a rigid linear array of (two or three) spherical particles. The direct pair interactions between all the spheres in the system (including those of the tracer particle) are modeled by a repulsive Yukawa potential. For simplicity, the twodimensional version of this simulation experiment is considered, and hydrodynamic interactions are ignored. From the simulation experiment, we determine the translational and rotational meansquaredisplacement of the nonspherical tracer particle. Here we focus only on the early deviations, due to the direct interactions, from the shorttime, freediffusion regime. In the analysis of these results, use is made of the recently developed Generalized Langevin equation theory of tracer diffusion of nonspherical Brownian particles.

Intermediatetime tracerdiffusion of nonspherical Brownian particles
View Description Hide DescriptionThe timedependent tracerdiffusion properties of a nonspherical Brownian particle that interacts with a suspension of spherical particles are studied in terms of an idealized but nontrivial model system for which the predictions of the generalized Langevin equation approach to tracer diffusion can be calculated, and compared with the results of a computer simulationexperiment. In the model, the nonspherical particle is represented by a linear array of (=2 or 3) spherical particles with nearestneighbor separation For this model, we calculate the rotational and the (transversal and longitudinal) translational mean squared displacements, both, directly from the computer simulation, and approximately using the generalized Langevin equation approach. The theory is found to reproduce qualitatively and quantitatively the main features of the results of the simulation experiment for these properties.

Dynamic correlation effect in reversible diffusioninfluenced reactions: Brownian dynamics simulation in three dimensions
View Description Hide DescriptionA Brownian dynamics (BD) simulation for a pseudofirstorder diffusioninfluenced reversible association–dissociation reaction of a target system in three dimensions with spherical symmetry is presented. The exact Green function for a reversible geminate dissociation that we obtained recently is utilized in the simulation. We compare the results of simulation with two successful theoretical predictions, the enhanced version of the superposition approximation approach (SA) and the more rigorous kinetic theoretical approach (KT). The KT predicts the correct power law behavior of with a slightly higher amplitude in the longtime region, but it is in good agreement with the BD result in the transient region. On the other hand, a faster relaxation is observed in the transient region for the SA, but the correct power law behavior with numerically exact amplitude is predicted for the exact target system. An interesting interplay between the mobility of the system and the dynamic correlation effect incorporated with manybody problems is also revealed.

A firstprinciple computation of the thermodynamics of glasses
View Description Hide DescriptionWe propose a firstprinciple computation of the equilibrium thermodynamics of simple fragile glasses starting from the twobody interatomic potential. A replica formulation translates this problem into that of a gas of interacting molecules, each molecule being built of m atoms, and having a gyration radius (related to the cage size) which vanishes at zero temperature. We use a small cage expansion, valid at low temperatures, which allows to compute the cage size, the specific heat (which follows the Dulong and Petit law), and the configurational entropy.

Simulation studies of liquid ammonia by classical ab initio, classical, and pathintegral molecular dynamics
View Description Hide DescriptionThe structure of liquid ammonia at T=273 K has been studied using classical ab initiomolecular dynamics, classical molecular dynamics, and the pathintegral molecular dynamics methods. The three different types of calculation are employed to generate new insights into the ability of theoretical methods to model liquid ammonia effectively. Thus, the limitations of using classical nuclei, simple point charge models, small systems, and gradient corrected density functional theory are assessed through a comparison of the results of the different types of calculations to each other and recent experiments in a consistent manner. Briefly, the experimental intermolecular quantum structure is very well reproduced by the classical approximation while the intramolecular classical and quantum structures exhibit large deviations. The intermolecular ab initio partial radial structure factors of liquid ammonia and the associated radial distribution functions are in better agreement with experiment than the empirical models. However, the empirical models also perform reasonably well.

Identifying physical clusters in bubble nucleation
View Description Hide DescriptionWe present a new approach to molecular simulation of bubble nucleation. Our approach does not involve any ad hoc criteria to define a bubble for a given instantaneous configuration of molecules. Instead, we explore the stochastic evolution of a system chosen as a small part of the liquid phase by means of an isothermal–isobaric Monte Carlo simulation aided by the umbrella sampling technique. The physical clusters relevant to nucleation, bubbles in the present case, emerge naturally as we attain a coarsegrained description of this stochastic process by introducing proper order parameters, i.e., the volume and the interaction potential of the system. Thus, the concept of cluster commonly employed to describe vapor to liquid nucleation is generalized naturally for the case of bubble nucleation. The method is applied to LennardJones fluids to evaluate the free energy of bubble formation under a moderate negative pressure. The interaction potential plays a similar role to that in vapor to liquid nucleation in that it characterizes the spatial extent of the bubble. There is thus a unity in free energetics of vapor to liquid nucleation and of bubble nucleation.

A primitive model of a charged hard ellipsoidal fluid
View Description Hide DescriptionA method is developed for the theoretical investigation of the structure of fluids comprised of hard nonspherical molecules carrying electric charge. The development is based upon the mean spherical approximation for the direct correlation function of such fluids. Since the equations based upon this approximation cannot in general be solved analytically an approximate ansatz for the direct correlation function containing a small number of free parameters is introduced. The free parameters are then determined from a standard variational principle. The ansatz itself is chosen to produce the known results in the strong coupling (large charge) limit and in the special case of hard spheres. In order to ensure that the solution in the limit of strong coupling has a tractable analytic form a new model is proposed for the charge distribution on an individual molecule. The method is applied in detail to a model of charged hard ellipsoids which is a generalization of the primitive model for ionic fluids and is found to be practical; it reduces to the primitive model in the special case of charged hard spheres. It is shown that the approximation preserves the conservation of charge.Properties investigated include the direct correlation function itself, the electrostatic energy of the fluid, the pair distribution function and the electrical potential surrounding an individual molecule. Results for these quantities are obtained for a range of densities and charge covering four orders of magnitude and for molecules with elongations (ratios of lengths of axes) from 0.5 (oblate) to 10 (prolate). The direct correlation function itself is given in an analytic form which can be used together with an appropriate density functional to investigate the structure of the electrical double layer formed by the fluid at a solid boundary.

Molecular dynamics simulation of formamide in water using density functional theory and classical potentials
View Description Hide DescriptionWe report the first molecular dynamics simulation of an amide in water in which the solute is fully described through quantum mechanics methods (density functional theory in our case). All solute’s degrees of freedom are allowed to vary. The solvent is described through a classical potential. We have chosen for our study the simple formamide molecule since it allows hybrid simulations to be carried out at a sophisticated quantum level. More precisely, we have considered two computational schemes: in the first one, we use a small doubleζ basis set and a local approximation of the exchangecorrelation functional whereas, in the second, an extended basis set, as well as a gradientcorrected functional, has been employed. The analysis of the results is focused on both structural and energetic aspects. Particular attention is paid to the time variation of dihedral angles in formamide connected to nitrogen pyramidalization and subunit rotation. The agreement with available experimental and theoretical data is satisfactory. Nevertheless, the limits of the method are pointed out, in particular the need to improve the description of the nonelectrostatic term of the solutesolvent interaction potential. One of the main advantages of the hybrid approach is that polarization effects are included in a rigorous manner. This renders possible a detailed discussion on the role of hydration effects on amides structure, a point of considerable relevance due to the biochemical importance of the peptidic bond.

Revisiting angular jump models of molecular reorientations in viscous liquids
View Description Hide DescriptionWe generalize the frequently used model of isotropic reorientational Brownian motion via small but finite angular steps to the case in which molecular reorientations by different angles take place around arbitrary axes. A simple approximation allows to give analytical expressions for experimentally relevant correlation functions, including spin lattice relaxation rates. We compare the results of our model to earlier treatments of rotational Brownian motion.

Multidimensional solvation dynamical effects on quantum yields in model triad systems
View Description Hide DescriptionThe dynamical effects on the quantum yield for the electronic state, (the final state) representing the efficiency of the system as an energy conversion system in a model triad system made out of an electron donor, D, an acceptor, A and a medium, M are investigated. The effective quantum yield for the final state is introduced, which is calculated only from the rates in a long time limit, and is applicable even when a memory effect from nuclear dynamics on the rates is considerable. Applying the effective quantum yield formalism, and taking into account multidimensional solvation dynamics by the theory previously developed by the authors, the yield for the final state, is calculated. Considerable dynamical effects are found in some specific situations. For example, the larger yield for the final state is obtained for the larger solvation time scale when the transfer from the initial state, to the final state occurs through the intermediate state, prior to nuclear thermalization in the intermediate state; whereas the similar transfer but with nuclear thermalization in the intermediate state is almost impossible.
