Roger Impey

Comparison of simple potential functions for simulating liquid water

Classical Monte Carlo simulations have been carried out for liquid water in the NPT ensemble at 25 °C and 1 atm using six of the simpler intermolecular potential functions for the water dimer: Bernal–Fowler (BF), SPC, ST2, TIPS2, TIP3P, and TIP4P. Comparisons are made with experimental thermodynamic and structural data including the recent neutron diffraction results of Thiessen and Narten. The computed densities and potential energies are in reasonable accord with experiment except for the original BF model, which yields an 18% overestimate of the density and poor structural results. The TIPS2 and TIP4P potentials yield oxygen–oxygen partial structure functions in good agreement with the neutron diffraction results. The accord with the experimental OH and HH partial structure functions is poorer; however, the computed results for these functions are similar for all the potential functions. Consequently, the discrepancy may be due to the correction terms needed in processing the neutron data or to an effect uniformly neglected in the computations. Comparisons are also made for self‐diffusion coefficients obtained from molecular dynamics simulations. Overall, the SPC, ST2, TIPS2, and TIP4P models give reasonable structural and thermodynamic descriptions of liquid water and they should be useful in simulations of aqueous solutions. The simplicity of the SPC, TIPS2, and TIP4P functions is also attractive from a computational standpoint.

Molecular model for aqueous ferrous–ferric electron transfer

We present a molecular model for studying the prototypical ferric–ferrous electron transfer process in liquid water, and we discuss its structural implications. Treatment of the nonequilibrium dynamics will be the subject of future work. The elementary constituents in the model are classical water molecules, classical ferric ions (i.e., Fe3+ particles), and a quantal electron. Pair potentials and pseudopotentials describing the interactions between these constituents are presented. These interactions lead to ligand structures of the ferric and ferrous ions that are in good agreement with those observed in nature. The validity of the tight binding model is examined. With umbrella sampling, we have computed the diabatic free energy of activation for electron transfer. The number obtained, roughly 20 kcal/mol, is in reasonable accord with the aqueous ferric–ferrous transfer activation energy of about 15 to 20 kcal/mol estimated from experiment. The Marcus relation for intersecting parabolic diabatic free ene...

Spectroscopic and transport properties of water

Computer simulation by the method of molecular dynamics has been used to study the dynamical properties of liquid water at several state points. The interactions between molecules were represented by the potential energy function of Matsuoka, Clementi and Yoshimine. The main effort has been directed at extracting from the simulations results of significance for the interpretation of measurements of spectroscopic and transport properties of real water, including isotope effects. Results are reported for the self diffusion coefficient, N.M.R. relaxation times, infrared and Raman spectra, and functions relevant to coherent and incoherent inelastic neutron scattering and to dielectric relaxation. A basis is provided for the explanation of a number of experimental observations.

On the infrared and Raman spectra of water in the region 5-250 cm−1

Abstract The far-infrared absorption coefficient and R (ω) representation of the Raman spectrum are calculated for the interaction-induced dipole and polarizability of water by computer simulation, using the MCY potential. A simple dipole-induced dipole model is able to reproduce experimentally observed bands at 60 and 200 cm −1 . It is shown that these bands may be associated with oscillations in the centre-of-mass velocity correlation function.

Molecular dynamics studies of the structure of water at high temperatures and density

The effect of changes in temperature and density on the structure of water has been studied by molecular dynamics using the analytical potential of Matsuoka, Clementi, and Yoshimine. Significant structural changes are found to occur, particularly at high density. Implications for the interpretation of neutron scattering experiments are discussed.

Molecular dynamic simulation of quadrupole relaxation of atomic ions in aqueous solution

Molecular dynamics (MD) simulations were performed in order to investigate the molecular origin of the nuclear quadrupole relaxation mechanism for Li+, Na+, and Cl− ions in dilute aqueous solution. Different boundary conditions were investigated in the simulations, but neither the boundary conditions nor the system size have any significant effect on the different time correlation functions (tcf:s) calculated. It is found that the field gradient tcf, determining the NMR relaxation rate for these ions, is almost completely due to the water molecules in the first shell. The calculated field gradient tcf:s show a complex time‐dependence not describable with a mono‐exponential decay. The very rapid decay shown by the field gradient tcf for all three ions can partly be attributed to a correlated water motion in the first hydration shell. The relaxation rates obtained from the simulations are in excellent agreement with experimental data for Na+ and Cl−, while the Li+ relaxation rate is less well produced.

A simple intermolecular potential for liquid ammonia

Abstract A simple intermolecular potential function is derived for liquid ammonia that is suitable for computer simulations. The potential is shown to yield reasonable thermodynamic and structural data. The model is further tested by carrying out molecular dynamics calculations on solid ammonia.

Study of electron solvation in liquid ammonia using quantum path integral Monte Carlo calculations

The solvation of an electron in liquid ammonia has been studied using quantum path integral Monte Carlo calculations. In agreement with previous experimental and theoretical deductions the charge distribution of the electron is compact. Various distribution functions characterizing the structure around the solvated electron are presented and the surrounding solvent structure is compared to that around a classical atomic anion. A qualitative discussion is given of the absorption spectrum based upon the form of the complex time dependence of the electron mean squared displacement correlation function.

Structural and dynamic properties of lithium sulphate in its solid electrolyte form

The properties of solid lithium sulphate have been studied by computer simulation. At sufficiently high temperatures, the simulated crystal behaves as a solid electrolyte with lithium ion (jump) diffusion and sulphate group rotation. The atomic radial distribution functions in the rotator phase are discussed in relation to the low temperature, fully ordered, monoclinic structure and the nature of the orientational disorder of the sulphate groups is characterized in terms of tetrahedral rotor functions. The crystal structure factor is found to be sensitive to the model adopted for the charge distribution of the anions; good agreement with experimental neutron diffraction data is obtained when a charge distribution consistent with ab initio quantum mechanical calculations is used. The phase transition whereby the low temperature monoclinic structure transforms to the disordered cubic phase has been investigated by the constant pressure molecular dynamics method. The nature of the lithium ion diffusive motio...

GridX1: A Canadian computational grid

The present paper discusses the design and application of GridX1, a computational grid project which uses shared resources at several Canadian research institutions. The infrastructure of GridX1 is built using off-the-shelf Globus Toolkit 2 middleware, a MyProxy credential server, and a resource broker based on Condor-G to manage the distributed computing environment. The broker-based job scheduling and management functionality are exposed as a Globus GRAM job service. Resource brokering is based on the Condor matchmaking mechanism, whereby job and resource attributes are expressed as ClassAds, with the attributes Requirements and Rank being used to define respectively the constraints and preferences that the matched entity must meet. Various strategies for ranking resources are presented, including an Estimated-Waiting-Time (EWT) algorithm, a throttled load balancing strategy, and a novel external ranking strategy based on data location. One of the unique features is a mechanism which transparently presents the GridX1 resources as a single compute element to the LHC Computing Grid (LCG), based at the CERN Laboratory in Geneva. This interface was used during the ATLAS data challenge 2 to federate the Canadian resources into the LCG without the overhead of maintaining separate LCG sites. Further, the BaBar particle physics simulation has been adapted to execute on GridX1 and resulted in a simplified management of the production. The usage of the throttled EWT and load balancing strategies combined with external data ranking was found to be very effective in improving efficiency and reducing the job failure rate.