J. W. Halley

A polarizable, dissociating molecular dynamics model for liquid water

We describe a molecular dynamics model for dissociable, polarizable water. The model, which describes both the static and dynamic properties of real water quite reasonably, contains the following features: Self‐consistent local fields are calculated in an extension of an earlier algorithm in which the dipole moments of the water are treated as dynamical variables. An intramolecular three‐body potential assures that the molecular properties of water are in agreement with experiment. Ewald methods are used to take account of monopole–dipole and dipole–dipole as well as monopole–monopole interactions. The model was optimized using a Monte Carlo procedure in the parameter space which is described.

Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of α-Fe2O3

We present a clear and rigorous derivation of the Ewald-like method for calculation of the electrostatic energy of the systems infinitely periodic in two dimensions and of finite size in the third dimension (slabs). We have generalized this method originally developed by Rhee et al. (Phys. Rev. B 40 (1989) 36) to account for charge-dipole and dipole-dipole interactions and therefore made it suitable for treatment of polarizable systems. This method has the advantage over exact methods of being significantly faster and therefore appropriate for large-scale molecular dynamics simulations. However, it involves a Taylor expansion which has to be demonstrated to be of sufficient order. The method was extensively benchmarked against the exact methods by Leckner and Parry. We found it necessary to increase the order of the multipole expansion from 4 (as in the original work by Rhee et al.) to 6. In this case the method is adequate for aspect ratios (thickness/shortest side length of the unit cell) ≤ 0.5. Molecular dynamics simulations using the transferable/polarizable model by Rustad et al. were applied to study the surface relaxation of the nonhydroxylated, hydroxylated and solvated surfaces of α-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreement with previous LDA calculations on α-alumina by Manassidis et al. (Surf. Sci. 285 (1993) L517). Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated-hydrated states for the surfaces which are used in energy minimization calculations. We find that hydration has a small effect on the surface structure, but that hydroxylation has a significant effect. Our calculations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the α-Fe2O3 (012) surface than for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface.

Molecular dynamics simulation of water beween two ideal classical metal walls

We have simulated a slab of water with two‐dimensional periodic boundary conditions between two metallic walls. The entire compliment of charges, arising from periodic reproductions and from classical images in the metal, are included explicitly by mapping onto a problem with three‐dimensional periodicity which is handled by usual Ewald summation methods. Results are presented for charged and uncharged surfaces, permitting an estimate of the differential capacitance arising from the layer of water near the walls. The estimate is about a factor of 2 smaller than the observed differential capacitance of metal–aqueous electrolyte interfaces.

Molecular dynamics, density functional theory of the metal–electrolyte interface

Quantitative, predictive theories for metal–electrolyte interfaces require an atomic‐scale representation of the interface, which must include an accurate statistical description of a polar fluid in contact with a solid surface; and also a description of the electronic density and structure of a metal surface in contact with a fluid. Such a complex system presents a difficult computational problem, and has been dealt with in the past essentially by parts; either by molecular dynamics calculations of the fluid structure, or density functional calculations of the metal–surface electronic structure. A complete and self‐consistent determination of the surface structure would, however, involve a simultaneous calculation of both the atomic and electronic structure of the interface. This suggests a combination of these two calculational techniques, and it is just this sort of molecular dynamics and density functional combination which comprises the Car–Parrinello, and related, methods. We have developed a Car–Pa...

A new model of the differential capacitance of the double layer

Abstract We present a new model of the metal-electrolyte interface which takes explicit account of the mobility of electrons at a metallic interface. The model can be described as a simple modification of the classical Stern-Gouy-Chapman double layer theory, but it contains an essential new feature of fundamentally quantum mechanical origin. A basic prediction of the theory is that at high enough temperatures the inverse 1/ C c of the compact part of the differential capacitance C c is linear in the charge σ m per unit area for small σ m if adsorption does not occur. We confirm this for Hg, In, Ag, Cd and (Pb) with NaF (KF) electrolyte. The slope of the 1/ C c vs. σ m line is predicted in a simple model to be proportional to the inverse 1/ϱ m of the metallic charge density ϱ m and we predict the coefficient on the basis of two detailed models. The results are compared with experiment on a variety of metal electrolyte interfaces. The limitations of the theory are discussed.

Molecular dynamics simulation of iron(III) and its hydrolysis products in aqueous solution

A simple potential model is described which allows molecular dynamics simulations to be performed for ferric iron ions in dissociating aqueous solutions. The model was parametrized by fitting the polarizable dissociating water model of Halley et al. [J. Chem. Phys. 98, 4110 (1993)] to a single water molecule–ferric iron ion potential energy surface taken from the work of Curtiss et al. [J. Chem. Phys. 86, 2319 (1987)]. The model gives very good results for the structure of the solvated hexaaqua iron(III) complex; the proper coordination number of 6 was obtained when the Fe–O interaction was fit directly to the ab initio calculations without further modification. The model produces adequate results for the first hydrolysis constant, but breaks down for the second hydrolysis constant, which is overestimated by 18 kcal/mol.

Atomic structure of solid and liquid polyethylene oxide

The structure of polyethylene oxide (PEO) was investigated by neutron scattering in both semicrystalline and liquid states. Deuterated samples were studied in addition to the protonated ones in order to avoid the large incoherent scattering of hydrogen and identify features in the pair correlation functions attributable to C–H pairs. Analysis of the deuterated sample gave additional information on the C–O and C–C pairs. The results are compared with molecular-dynamics simulations of liquid PEO.

A molecular dynamics model of the amorphous regions of polyethylene oxide

We report a molecular dynamics model of the amorphous regions of polyethylene oxide for use in study of the ionic transport mechanisms in this polymer when it is used as an electrolyte in batteries. The model is produced by beginning with a molecular dynamics of dimethyl ether which we have reported earlier and ‘‘polymerizing’’ it computationally by successively choosing pairs of unbonded methyl groups and changing the forces to describe the chemical bond in the polymer. This is not intended to be a fully realistic simulation of the actual chemical polymerization process, but we argue that it produces a useful model of a sample of the amorphous polymer. We report structural and dynamical properties of the resulting model, in which we have adopted torsion forces reported by Krimm and coworkers to fit the observed vibrational spectrum.

Molecular dynamics studies of complexing in binary molten salts with polarizable anions: MAX4

Anion polarizations have been introduced in a molecular dynamics simulation of a complexing ionic liquid MAX4 similar to tetrachloroaluminates. The influence of the polarizability on the structure of the melt has been deduced from the radial distribution functions, the numbers of nearest and next nearest neighbor pairs, the distribution of coordination numbers, and the angular distribution of various triplets. Surprisingly, A2X6 molecular species, similar to those postulated to exist in acid haloaluminate melts, were detected in our simulated ‘‘neutral’’ melt. The stability of other species (e.g., A2X−7 and A3X−10) that were present in the simulated MAX4 melt with nonpolarizable X anions decreased when polarization was introduced.

Simulation study of the ferrous ferric electron transfer at a metal--aqueous electrolyte interface

We report a new simulation study of the rate of ferrous–ferric electron transfer at a metal electrolyte interface. In contrast with earlier work, new features in our study include a detailed account of the effects of the field associated with the charging of the electrode, inclusion of entropic effects in the calculated free energy barriers, and a study of the dependence of the relevant free energy surfaces on the distance of the ion from the electrode. The qualitative picture of the reaction mechanism which emerges is significantly more detailed than that in earlier work. The dominant factors in determining the rate and mechanisms of electron transfer are the distance dependence of the work function of the metal, the redox species concentration profile, and the electronic matrix element. Calculated free energy barriers are consistent with experimentally measured ones. We also estimate the equilibrium potential for this reaction from the model, and find it to be consistent with the experimental equilibriu...