We investigate pairwise electrostatic interaction methods and show that there are viable computationally efficient (O(N)) alternatives to the Ewald summation for typical modern molecular simulations. These methods are extended from the damped and cutoff-neutralized Coulombic sum originally proposed by Wolf et al. [J. Chem. Phys. 110, 8255 (1999)]. One of these, the damped shifted force method, shows a remarkable ability to reproduce the energetic and dynamic characteristics exhibited by simulations employing lattice summation techniques. Comparisons were performed with this and other pairwise methods against the smooth particle-mesh Ewald summation to see how well they reproduce the energetics and dynamics of a variety of molecular simulations.

We report on systematic studies of size-dependent alloy formation of silver-coated gold nanoparticles (NPs) in aqueous solution at ambient temperature using X-ray absorption fine structure spectroscopy (XAFS). Various Au-core sizes (2.5-20 nm diameter) and Ag shell thicknesses were synthesized using radiolytic wet techniques. The equilibrium structures (alloy versus core-shell) of these NPs were determined in the suspensions. We observed remarkable size dependence in the room temperature interdiffusion of the two metals. The interdiffusion is limited to the subinterface layers of the bimetallic NPs and depends on both the core size and the total particle size. For the very small particles (< or =4.6 nm initial Au-core size), the two metals are nearly randomly distributed within the particle. However, even for these small Au-core NPs, the interdiffusion occurs primarily in the vicinity of the original interface. Features from the Ag shells do remain. For the larger particles, the boundary is maintained to within one monolayer. These results cannot be explained either by enhanced self-diffusion that results from depression of the melting point with size or by surface melting of the NPs. We propose that defects, such as vacancies, at the bimetallic interface enhance the radial migration (as well as displacement around the interface) of one metal into the other. Molecular dynamics calculations correctly predict the activation energy for diffusion of the metals in the absence of vacancies and show an enormous dependence of the rate of mixing on defect levels. They also suggest that a few percent of the interfacial lattice sites need to be vacant to explain the observed mixing.

The technique of molecular beam photofragment translational spectroscopy has been used to study the dissociation of acetone following S 1 ←S 0 ͑248 nm͒ and S 2 ←S 0 ͑193 nm͒ excitation. Excitation at 248 nm resulted in the production of CH 3 and CH 3 CO with 14.2Ϯ1.0 kcal/mole on average of the available energy appearing as translation of the photofragments. Comparison of the measured ͗E T ͘ with values reported at 266 nm suggest that the energy partitioning is dominated by the exit barrier caused by an avoided crossing on the potential energy surface. A substantial fraction ͑30Ϯ4%͒ of the nascent acetyl radicals from the primary dissociation contain sufficient energy to undergo spontaneous secondary decomposition. From the onset of the truncation of the CH 3 CO P(E T) a threshold of 17.8Ϯ3.0 kcal/mole for the dissociation of the acetyl radical has been determined in agreement with recent results on the photodissociation of acetyl chloride. The translational energy release in the dissociation of CH 3 CO closely matches the experimentally determined exit barrier. At 193 nm the only observed dissociation pathway was the formation of two methyl radicals and carbon monoxide. On average ϳ38% of the available energy is found in product translation suggesting that significant internal energy resides in the nascent CH 3 fragments consistent with the results of Hall et al. ͓J. Chem. Phys. 94, 4182 ͑1991͔͒. We conclude that the dynamics and energy partitioning for dissociation at 193 nm is similar to that at 248 nm.

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We present a method for estimating the hopping rate for Zwanzig's model of self-diffusion in liquids ͓R. Zwanzig, J. Chem. Phys. 79, 4507 ͑1983͔͒. To obtain this estimate, we introduce the cage correlation function which measures the rate of change of atomic surroundings, and associate the long-time decay of this function with the basin hopping rate for diffusion. Results from a set of simulations on Lennard-Jones particles are presented. A simple analytic model for the diffusion constant in supercooled and normal liquids that is based on estimates of the activation energy obtained via the cage correlation function is derived. We discuss the breakdown of Zwanzig's hopping mechanism for mass transport as well as the low temperature behavior of the self-diffusion constant on rough potential energy surfaces.

With the non-isotropic velocity scaling (NIVS) approach to reverse non-equilibrium molecular dynamics (RNEMD), it is possible to impose an unphysical thermal flux between different regions of inhomogeneous systems such as solid/liquid interfaces. We have applied NIVS to compute the interfacial thermal conductance at a metal/organic solvent interface that has been chemically capped by butanethiol molecules. Our calculations suggest that coupling between the metal and liquid phases is enhanced by the capping agents, leading to a greatly enhanced conductivity at the interface. Specifically, the chemical bond between the metal and the capping agent introduces a vibrational overlap that is not present without the capping agent, and the overlap between the vibrational spectra (metal to cap, cap to solvent) provides a mechanism for rapid thermal transport across the interface. Our calculations also suggest that this is a nonmonotonic function of the fractional coverage of the surface, as moderate coverages allow diffusive heat transport of solvent molecules that have been in close contact with the capping agent.

DFT investigations of structural and energetical features of [H2O2] n (n=−1,0,1) potential energy surfaces AIP Conf. Proc. 330, 197 (1995); 10.1063/1.47664 Structures in the energy dependence of the rate constant for ketene isomerization J. Chem. Phys. 98, 7846 (1993); 10.1063/1.464592 Predicting observables on different potential energy surfaces using feature sensitivity analysis: Application to the collinear H+H2 exchange reaction J. Chem. Phys. 97, 6240 (1992); 10.1063/1.463685An examination of the 21 A 1 states of formaldehyde and ketene including analytic configuration interaction energy first derivatives for singlet excited electronic states of the same symmetry as the ground state Calculations of the microcanonical isomerization rates for vibrationally excited ketene are presented. The calculations utilize the quantum reactive scattering methodology of absorbing boundary conditions with a discrete variable representation to obtain the cumulative reaction probability for one form of ketene to isomerize via the oxirene intermediate, and were carried out with model 1-, 2-, and 3-degree-of-freedom potential energy surfaces constructed using ab initio data. Significant differences are seen in the energy dependent features of the microcanonical rate for the single mode and multi-mode potentials; e.g., the single mode potential exhibits tunneling resonances with widths of around 1 cm Ϫ1 , while the calculations involving more than one degree of freedom have additional resonant features that have widths around 10 cm Ϫ1 and also exhibit non-Breit-Wigner resonant line shapes. This suggests that many of the resonance features are best described as Feshbach ͑energy transfer, or dynamical͒ resonances that result because of a strongly bent region on the multi-mode potential energy surfaces. The calculated rates show reasonable qualitative agreement with the experimental results of Lovejoy and Moore ͓J. Chem. Phys. 98, 7846 ͑1993͔͒.

We present an algorithm for carrying out Langevin dynamics simulations on complex rigid bodies by incorporating the hydrodynamic resistance tensors for arbitrary shapes into an advanced rotational integration scheme. The integrator gives quantitative agreement with both analytic and approximate hydrodynamic theories for a number of model rigid bodies and works well at reproducing the solute dynamical properties (diffusion constants and orientational relaxation times) obtained from explicitly solvated simulations.