Xueyu Song

The melting lines of model systems calculated from coexistence simulations

We have performed large-scale molecular dynamics simulations of coexisting solid and liquid phases using 4e(σ/r)n interactions for n=9 and n=12, and for Lennard-Jones systems, in order to calculate the equilibrium melting curve. The coexisting systems evolve rapidly toward the melting temperature. The P–T melting curves agree well with previous calculations, as do the other bulk phase properties. The melting curve for the Lennard-Jones system, evaluated using various truncations of the potential, converges rapidly as a function of the potential cutoff, indicating that long-range corrections to the free energies of the solid and liquid phases very nearly cancel. This approach provides an alternative to traditional methods of calculating melting curves.

The anisotropic free energy of the Lennard-Jones crystal-melt interface

We have calculated the free energy of the crystal-melt interface for the Lennard-Jones system as a function of crystal orientation, near zero pressure, by examining the roughness of the interface using molecular dynamic simulations. The anisotropy is weak, but can be accurately resolved using this approach due to the sensitivity of the fluctuations on the anisotropy. We find that the anisotropy can be described well using two parameters, based upon a low-order expansion satisfying cubic symmetry. The results are in good agreement with previous calculations of the free energies, based upon simulations used to calculate the reversible work required to create the interfaces. The weak anisotropy is also in reasonable agreement: The work here and the work of Davidchack and Laird [R. L. Davidchack and B. B. Laird, J. Chem. Phys. 118, 7651 (2003)] both predict γ100>γ110>γ111. The only discrepancy is that we find a smaller value for the difference γ100−γ111 by an amount larger than the combined error bars.

Quantum correction for electron transfer rates. Comparison of polarizable versus nonpolarizable descriptions of solvent

The electron transfer rate constant is treated using the spin-boson Hamiltonian model. The spectral density is related to the experimentally accessible data on the dielectric dispersion of the solvent, using a dielectric continuum approximation. On this basis the quantum correction for the ferrous–ferric electron transfer rate is found to be a factor 9.6. This value is smaller than the corresponding result (36) of Chandler and co-workers in their pioneering quantum simulation using a molecular model of the system [J. S. Bader, R. A. Kuharski, and D. Chandler, J. Chem. Phys. 93, 230 (1990)]. The likely reason for the difference lies in use of a rigid water molecular model in the simulation, since we find that other models for water in the literature which neglect the electronic and vibrational polarizability also give a large quantum effect. Such models are shown to overestimate the dielectric dispersion in one part of the quantum mechanically important region and to underestimate it in another part. It will be useful to explore a polarizable molecular model which reproduces the experimental dielectric response over the relevant part of the frequency spectrum.

Dielectric solvation dynamics of molecules of arbitrary shape and charge distribution

A new perspective of dielectric continuum theory is discussed. From this perspective a dynamical generalization of a boundary element algorithm is derived. This generalization is applied to compute the solvation dynamics relaxation function for chromophores in various solvents. Employing quantum chemical estimates of the chromophore’s charge distribution, the Richards–Lee estimate of its van der Waals surface, and the measured frequency dependent dielectric constant of the pure solvent, the calculated relaxation functions agree closely with those determined by experiments.

An inhomogeneous model of protein dielectric properties: Intrinsic polarizabilities of amino acids

A simple inhomogeneous model of protein dielectric properties is discussed. A protein in solution is modeled as a collection of polarizable dipoles in a cavity embedded inside a dielectric medium. The intrinsic polarizabilities of 20 amino acids are assumed to be portable to all proteins in nature. A reasonable set of these polarizability values has been obtained by comparing dielectric fluctuations from molecular dynamics simulations with model calculations. The results are consistent within a data set of three small proteins.

Solvation dynamics in ionic fluids: An extended Debye-Hückel dielectric continuum model

Motivated by our recent proposition on the possibility of using dielectric continuum models to interpret experimental measurements of solvation dynamics in room temperature ionic liquids [J. Phys. Chem. A 110, 8623 (2006)], some detailed simulation studies are performed to test the validity of our proposition. From these simulation studies, it seems to be justified that an extended Debye-Hückel continuum model can be used to understand the solvation dynamics of ionic fluids. The theoretical underpinning of such an extended Debye-Hückel model is presented from the general dispersion relation in electrodynamics. The connection with the static extension from the dressed ion theory of electrolyte solutions is also discussed. Such a connection between the Debye-Hückel theory and the dispersion relation may be exploited to enhance our understanding of the electric double layer problem not only for the static case but also for dynamic situations.

Outer-sphere Electron Transfer in Polar Solvents: Quantum Scaling of Strongly Interacting Systems.

The spin–boson Hamiltonian model is used to study electron transfer (ET) reactions of strongly interacting systems in polar solvents in the limit of fast dielectric relaxation of the solvent. The spectrum of polarization modes consists of low frequency modes which are treated classically, and high frequency modes which are treated quantum mechanically. A general explicit formula for the rate valid in all orders of perturbation theory in electronic coupling is derived. The rate formula is applicable in a wide range of parameters, including the inverted region of the reaction where the quantum tunneling corrections give the main contribution to the rate. It is found that the quantum degrees of freedom can be effectively eliminated from the model by renormalizing the electronic coupling matrix element. This renormalization results in the following scaling property of the electron transfer systems: a system containing both classical and quantum degrees of freedom is equivalent to a system of lower dimensional...

A molecular Debye-Hückel theory and its applications to electrolyte solutions

In this report, a molecular Debye-Hückel theory for ionic fluids is developed. Starting from the macroscopic Maxwell equations for bulk systems, the dispersion relation leads to a generalized Debye-Hückel theory which is related to the dressed ion theory in the static case. Due to the multi-pole structure of dielectric function of ionic fluids, the electric potential around a single ion has a multi-Yukawa form. Given the dielectric function, the multi-Yukawa potential can be determined from our molecular Debye-Hückel theory, hence, the electrostatic contributions to thermodynamic properties of ionic fluids can be obtained. Applications to binary as well as multi-component primitive models of electrolyte solutions demonstrated the accuracy of our approach. More importantly, for electrolyte solution models with soft short-ranged interactions, it is shown that the traditional perturbation theory can be extended to ionic fluids successfully just as the perturbation theory has been successfully used for short-ranged systems.

Considerations for the Construction of the Solvation Correlation Function and Implications for the Interpretation of Dielectric Relaxation in Proteins

The dielectric response of proteins is conveniently measured by monitoring the time-dependent Stokes shift of an associated chromophore. The interpretation of these experiments depends critically upon the construction of the solvation correlation function, C(t), which describes the time-dependence of the Stokes shift and hence the dielectric response of the medium to a change in charge distribution. We provide an analysis of various methods of constructing this function and review selected examples from the literature. The naturally occurring amino acid, tryptophan, has been frequently used as a probe of solvation dynamics in proteins. Its nonexponential fluorescence decay has stimulated the generation of an alternative method of constructing C(t). In order to evaluate this method, we have studied a system mimicking tryptophan. The system is comprised of two coumarins (C153 and C152) having different fluorescence lifetimes but similar solvation times. The coumarins are combined in different proportions in methanol to make binary probe mixtures. We use fluorescence upconversion spectroscopy to obtain wavelength-resolved kinetics of the individual coumarins in methanol as well as the binary mixtures of 75:25, 50:50, and 25:75 of C153:C152. The solvation correlation functions are constructed for these systems using different methods and are compared.

Quantum effects in electron transfer reactions with strong electronic coupling

A new complex centroid reaction coordinate method is used to study electron transfer systems with strong electronic coupling. Formal analogy between current problem and the Ising model of one‐dimensional spin system is used to develop a useful approximation for the partition function of electron transfer system in all orders of perturbation theory and when quantum effects are present. The reactions in the inverted region are discussed. The range of applicability of the usual nonadiabatic theory is re‐examined. It is concluded that quantum solvent modes can effectively reduce electronic coupling in such a way that a nonadiabatic behavior can sometimes be induced in conventionally strongly coupled systems. Such an induced quantum nonadiabaticity is demonstrated in a numerical calculation.