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Richard M. Stratt

Bio: Richard M. Stratt is an academic researcher from Brown University. The author has contributed to research in topics: Solvation & Relaxation (physics). The author has an hindex of 35, co-authored 109 publications receiving 4411 citations. Previous affiliations of Richard M. Stratt include University of Southern California & University of Illinois at Urbana–Champaign.


Papers
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TL;DR: Two of the more fundamental ways in which molecules change their behavior when they are dissolved are that they can begin to exchange energy with the surrounding liquid and they can induce their surroundings to rearrange so as to provide a significant stabilizing influence as mentioned in this paper.
Abstract: Two of the more fundamental ways in which molecules change their behavior when they are dissolved are that they can begin to exchange energy with the surrounding liquid and they can induce their surroundings to rearrange so as to provide a significant stabilizing influence. The first of these is typified by the process of vibrational population relaxation of a vibrationally hot species. The second conceptcritical to solution chemistryis what is known as solvation. Both of these processes are sufficiently fundamental that one would really like to know, at the most mechanical and molecular level possible, just what events are required in order to make them happen. But how difficult is it going to be to extract such molecular detail from the complicated many-body dynamics? The most microscopic level of understanding one could ever hope to possess might seem far removed from the finely detailed dynamical information which is available routinely for individual isolated molecules and for molecule−molecule colli...

611 citations

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TL;DR: In this article, it is shown that the dynamics of diatomic liquids are governed by a set of independent, collective, harmonic modes of the liquid, and that the negative regions of both the translational and rotational velocity autocorrelation functions can be understood in terms of these same instantaneous harmonic modes.
Abstract: Since the sharply varying forces that control the arrangement of molecules in liquids are themselves intrinsically anharmonic, the natural assumption would be that any picture that regarded molecular motion as harmonic would be at best a rough phenomenological guide. This expectation is, in fact, not a correct one. While the packing forces that determine liquid structure are indeed strongly anharmonic, the short‐time displacements and librations that molecules execute are actually quite harmonic. It is possible to show rigorously that, for short enough (subpicosecond) time intervals, the dynamics of liquids is governed by a set of independent, collective, harmonic modes—the instantaneous normal modes of the liquid. In this paper we illustrate this fact by predicting the translational and rotational dynamics of a model diatomic liquid using the instantaneous normal modes computed by simulation. When compared to the exact molecular‐dynamics results for the same autocorrelation functions, we find that perfect agreement is maintained only for very short times, but that if one removes the artificial runaway dynamics caused by the imaginary‐frequency modes, reasonable levels of agreement are maintained for much longer time intervals. We also investigate the nature of the coupled translational–rotational motion by looking at the relevant translational and rotational projections of the modes. We find that the negative (backscattering) regions of both the translational‐ and rotational‐velocity autocorrelation functions can be understood in terms of these same instantaneous harmonic modes.

202 citations

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TL;DR: This paper presents a general methodology to carry out the reduction in numbers of degrees of freedom in path integral algorithms, and shows how to use discretized path integrals to compute rigorous upper and lower bounds to the free energy for nontrivial quantum systems.
Abstract: In the path integral representation of quantum theory, a few body quantum problem becomes a classical many body problem. To exploit this isomorphism, it becomes necessary to develop methods by which degrees of freedom can be explicitly removed from consideration. The interactions among the remaining relevant variables are described by effective interactions. In this paper, we present a general methodology to carry out the reduction in numbers of degrees of freedom. Certain path integral algorithms are shown to correspond to reference systems for the full isomorphic classical many body problem. The correspondence allows one to determine systematic corrections to the algorithms by low order perturbation approximations familiar in the theory of simple classical fluids. We show how to use discretized path integrals to compute rigorous upper and lower bounds to the free energy for nontrivial quantum systems, and we discuss how to optimize the upper bounds with variational theories. Several illustrative example...

179 citations


Cited by
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TL;DR: In this paper, the authors report, extend, and interpret much of our current understanding relating to theories of noise-activated escape, for which many of the notable contributions are originating from the communities both of physics and of physical chemistry.
Abstract: The calculation of rate coefficients is a discipline of nonlinear science of importance to much of physics, chemistry, engineering, and biology. Fifty years after Kramers' seminal paper on thermally activated barrier crossing, the authors report, extend, and interpret much of our current understanding relating to theories of noise-activated escape, for which many of the notable contributions are originating from the communities both of physics and of physical chemistry. Theoretical as well as numerical approaches are discussed for single- and many-dimensional metastable systems (including fields) in gases and condensed phases. The role of many-dimensional transition-state theory is contrasted with Kramers' reaction-rate theory for moderate-to-strong friction; the authors emphasize the physical situation and the close connection between unimolecular rate theory and Kramers' work for weakly damped systems. The rate theory accounting for memory friction is presented, together with a unifying theoretical approach which covers the whole regime of weak-to-moderate-to-strong friction on the same basis (turnover theory). The peculiarities of noise-activated escape in a variety of physically different metastable potential configurations is elucidated in terms of the mean-first-passage-time technique. Moreover, the role and the complexity of escape in driven systems exhibiting possibly multiple, metastable stationary nonequilibrium states is identified. At lower temperatures, quantum tunneling effects start to dominate the rate mechanism. The early quantum approaches as well as the latest quantum versions of Kramers' theory are discussed, thereby providing a description of dissipative escape events at all temperatures. In addition, an attempt is made to discuss prominent experimental work as it relates to Kramers' reaction-rate theory and to indicate the most important areas for future research in theory and experiment.

5,180 citations

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TL;DR: In this paper, an algorithm, called RATTLE, for integrating the equations of motion in molecular dynamics calculations for molecular models with internal constraints is presented. But it is based on the Verlet algorithm and retains the simplicity of using Cartesian coordinates for each of the atoms to describe the configuration of a molecule with internal constraint.

2,669 citations

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TL;DR: In this paper, the authors introduce a picture of a boson superfluid and show how superfluidity and Bose condensation manifest themselves, showing the excellent agreement between simulations and experimental measurements on liquid and solid helium for such quantities as pair correlations, the superfluid density, the energy, and the momentum distribution.
Abstract: One of Feynman's early applications of path integrals was to superfluid $^{4}\mathrm{He}$. He showed that the thermodynamic properties of Bose systems are exactly equivalent to those of a peculiar type of interacting classical "ring polymer." Using this mapping, one can generalize Monte Carlo simulation techniques commonly used for classical systems to simulate boson systems. In this review, the author introduces this picture of a boson superfluid and shows how superfluidity and Bose condensation manifest themselves. He shows the excellent agreement between simulations and experimental measurements on liquid and solid helium for such quantities as pair correlations, the superfluid density, the energy, and the momentum distribution. Major aspects of computational techniques developed for a boson superfluid are discussed: the construction of more accurate approximate density matrices to reduce the number of points on the path integral, sampling techniques to move through the space of exchanges and paths quickly, and the construction of estimators for various properties such as the energy, the momentum distribution, the superfluid density, and the exchange frequency in a quantum crystal. Finally the path-integral Monte Carlo method is compared to other quantum Monte Carlo methods.

1,908 citations

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TL;DR: In this paper, a qualitative discussion of electron transfer, its time and distance scales, energy curves, and basic parabolic energy models are introduced to define the electron transfer process, and some of the important, challenging, and problematic issues in contemporary electron transfer research are discussed.
Abstract: This is an overview of some of the important, challenging, and problematic issues in contemporary electron transfer research. After a qualitative discussion of electron transfer, its time and distance scales, energy curves, and basic parabolic energy models are introduced to define the electron transfer process. Application of transition state theory leads to the standard Marcus formulation of electron transfer rate constants. Electron transfer in solution is coupled to solvent polarization effects, and relaxation processes can contribute to and even control electron transfer. The inverted region, in which electron transfer rate constants decrease with increasing exoergicity, is one of the most striking phenomena in electron transfer chemistry. It is predicted by both semiclassical and quantum mechanical models, with the latter appropriate if there are coupled high- or medium-frequency vibrations. The intramolecular reorganizational energy has different contributions from different vibrational modes, whic...

1,413 citations