Short-range Term Research Articles

A novel approach for designing simple point charge models for liquid water with three interaction sites

A simultaneous improvement of the diffusion and dielectric properties of the simple point charge (SPC) model for liquid water appears to be very difficult with conventional reparametrization of the commonly used Lennard-Jones and Coulomb interaction functions and without including a self-energy correction in the effective pair-potential as is done in the SPC/E model. Here, a different approach to circumvent this problem is presented. A short-range interaction term, which corrects the oxygen-oxygen energy at small distances by small amounts of energy, was introduced in the nonbonded interaction function. This additional force-field term allows to derive new parameter sets for SPC-like water models that yield better agreement with experimental data on liquid water. Based on previous investigations of the force-field parameter dependence of the water properties of SPC-like models, the necessary parameter changes to obtain a lower diffusion coefficient and a larger dielectric permittivity were specified and accordingly six new models were developed. They all represent an improvement over SPC in terms of structural and diffusional properties, four of them show better dielectric properties also. One model, SPC/S, has been characterized in more detail, and represents most properties of liquid water better than SPC while avoiding the larger discrepancies with experimental values regarding density, thermal compressibility, energy, and free energy of the SPC/E model. We conclude that the use of a simple, short-ranged additional oxygen-oxygen interaction term makes a simultaneous improvement of the diffusion coefficient and the dielectric properties of water feasible.

On the Correlation of the Activity Coefficients in Aqueous Electrolyte Solutions Using the K-MSA Model

The objective of this work is to extend the Kelvin mean spherical approximation (K-MSA) model to correlate the mean and individual ionic activity coefficients for symmetric and asymmetric electrolyte solutions at different concentrations and at different temperatures. Revised values of the parameters for the mean and individual ionic activity coefficients of 1:1 electrolytes and new values of the parameters for the mean ionic activity coefficients of asymmetric electrolytes are presented. The effect of the short-range electrostatic term (Pn) at different size ratios of the ions is examined. Assuming a constant anion diameter and a composition-dependent diameter for the cation, the K-MSA model gives realistic cation-hydrated diameters, in comparison with their crystallographic Pauling diameters. Notably, for both symmetric and asymmetric electrolytes, the like and unlike ionic radial distribution function at contact value are positive over all of the concentration and temperature ranges studied. The result...

Ab initio three-body interactions for water. I. Potential and structure of water trimer

A new ab initio three-body potential for water has been generated from the Hartree–Fock method and symmetry-adapted perturbation theory calculations performed at 7533 trimer geometries. The calculated nonadditive energies were then fitted to a physically motivated analytic formula containing representations of short-range exchange contributions and damped induction terms. To our knowledge, this is the first time the short-range nonadditive interactions have been explicitly included in a potential for water. The fitted nonadditive potential was then applied, together with an accurate ab initio pair potential, SAPT-5s, to evaluate the effects of nonadditivity on the structure and energetics of water trimer.

A Microscopic Description of Concentrated Potassium Fluoride Aqueous Solutions by Molecular Dynamics Simulation

We describe the microscopic structure and dynamical properties of potassium fluoride aqueous solutions at T = 298 K as a function of concentration up to 11.9 M. All calculations are made using constant temperature and pressure as well as microcanonical molecular dynamics simulations on the basis of an optimized pair potential model which is a mix of Coulomb plus standard Lennard-Jones short-range terms for water and ion-water interactions and Tosi-Fumi potential for ion-ion interactions. In particular we propose a modified ion-water interaction that successfully reproduces the experimental behavior of the density and water self-diffusion of the solutions as a function of concentration. Finally, we analyze hydrogen-bonding in solutions looking at pair distribution energies and residence times. We find strong evidence for K + -F - pairing in solutions at the higher concentrations.

Apparent Molar Volume, Isentropic Compressibility, Refractive Index, and Viscosity of DL-Alanine in Aqueous NaCl Solutions

New experimental data at 25°C for the density, velocity of sound, refractive index, and viscosity of aqueous solutions of DL-alanine and NaCl are reported. The apparent molar volume and isentropic compressibility of DL-alanine in aqueous electrolyte solutions have been calculated from the measured properties. The results show that DL-alanine exhibits a positive volume transfer to solutions of a higher NaCl concentration and a negative apparent isentropic compressibility for DL-alanine in the presence of NaCl. These effects indicate that the apparent volume of DL-alanine is larger in solutions with higher electrolyte concentration and the water molecules surrounding the DL-alanine molecules are less compressible than the water molecules in the bulk solution. The results also show an increase in the viscosity of the solution with an increase in both DL-alanine and NaCl concentrations. These effects are attributed to the two charged groups of DL-alanine and the interactions between the charged groups and the hydrocarbon backbone of DL-alanine with the ions. A model, consisting of a short-range interaction term represented by a virial expansion and a Debye-Huckel term that considers long-range interactions, has been developed to correlate the measured experimental data.

A speciation-based model for mixed-solvent electrolyte systems

A comprehensive model has been developed for the calculation of speciation, phase equilibria, enthalpies, heat capacities and densities in mixed-solvent electrolyte systems. The model incorporates chemical equilibria to account for chemical speciation in multiphase, multicomponent systems. For this purpose, the model combines standard-state thermochemical properties of solution species with an expression for the excess Gibbs energy. The excess Gibbs energy model incorporates a long-range electrostatic interaction term expressed by a Pitzer–Debye–Hückel equation, a short-range interaction term expressed by the UNIQUAC model and a middle-range, second virial coefficient-type term for the remaining ionic interactions. The standard-state properties are calculated by using the Helgeson–Kirkham–Flowers equation of state for species at infinite dilution in water and by constraining the model to reproduce the Gibbs energy of transfer between various solvents. The model is capable of accurately reproducing various types of experimental data for systems including aqueous electrolyte solutions ranging from infinite dilution to fused salts, electrolytes in organic or mixed, water+organic, solvents up to the solubility limit and acid–water mixtures in the full concentration range.

The effect of surface curvature on wound healing in bone: II. The critical size defect.

Two related models are studied for wound healing on a spherical surface. The first, and simpler model is based on a reaction-diffusion equation describing the process of healing as the inward growth of new cells from a circular wound on a spherical surface. Several aspects of the wound healing problem are considered: by interpreting the healing process as a “pseudowave” propagating across the spherical surface a heuristic account of the “speed” of healing is possible, and the corresponding healing time is characterized. Of particular importance in relation to animal models is the existence (or not) of a critical size defect (a CSD, defined below); this is discussed as a consequence of the stability of the steady states of the system to nonuniform spatial (or angular) perturbations. Explicit criteria are derived under which a CSD exists (within the model) in terms of the skull radius and wound radius. Although neither model explicitly distinguishes between wound healing in bone or tissue, it is in the former that CSDs are known to occur. The second model invokes a weighted spatial average cell density which permits the presence of both a short-range activation term (as in the first model) and a long-range inhibition term. Under these circumstances, within a suitable parameter range, the phenomenon of aggregation may occur in addition to the behavior predicted by the first model. It is suggested that such aggregation is manifested in the case of keloid scarring, which can occur as a result of wound healing in tissue.

A lattice dynamics model for M 2 XY 6 systems in the Fm 3 m ( O h 5 ) space group

This work is about the development of a generalized dynamic model to account for both the short-range and the long-range interactions in M2XY6-type crystals in cubic environments. The short-range vibrational interactions are handled using a mixed force field, whereas the long-range interaction terms are worked out by employing a revised version of the Ewald method. In both cases, new criteria are introduced so as to get a more realistic overall picture of the lattice dynamics for these crystals. As for the short-range vibrational terms, a criterion is included to minimize the cross terms in the potential-energy distribution and in this way to include the idea of a natural potential-energy distribution. The Coulombic interaction terms are worked out and a new convergence test is introduced to make sure that the series expansion in both the direct and the reciprocal spaces converge at the same speed and simultaneously, preserving the electroneutrality of the system. The current model has been applied to Cs2UBr6; we have fitted the k→=0→ vibrational frequencies as well as the generation of the phonon dispersion curves for different polarization directions. As a result, the model is shown to have some utility and flexibility to handle this kind of complex calculation and could be generalized to more complex systems, such as elpasolite-type crystals, to test the validity of the approximations and constrains involved. We anticipate that many questions are still open to discussion and the need for more complete and accurate data is emphasized throughout the course of the current research work.

How Well Does Microsolvation Represent Macrosolvation? A Test Case: Dynamics of Decarboxylation of 4-Pyridylacetic Acid Zwitterion

We present a systematic comparison of a cluster model to a continuum solvation model for the rate of decarboxylation of 4-pyridylacetic acid zwitterion. Geometries of the zwitterion and the transition state were optimized for the gas-phase reaction, for one model of reaction in aqueous solution in a previous study, and for five additional models of reaction in aqueous solution in the present study. The aqueous calculations include models in which one or two waters are treated explicitly, either without or with a surrounding continuum of bulk solvent; the latter is called a mixed discrete-continuum model. Bulk solvation effects are modeled both by solvation model SM5.42 and by a COSMO united atom solvation model; both of these models contain bulk electrostatics and short-range terms. Kinetic isotope effects were calculated by conventional transition state theory. We find that a discrete model of solvation with two explicit water molecules underestimates the free energy of activation for the decarboxylation reaction by at least 17 kcal/mol.

Monte Carlo Simulations of Small Sulfuric Acid−Water Clusters

Effective atom−atom potentials are developed for binary sulfuric acid−water clusters and applied in a Bennett Metropolis Monte Carlo calculation to determine free energy differences for small neighboring sized clusters of fixed composition at 298 K. The atom−atom pair potentials consist of Lennard-Jones short-range and Coulombic long-range terms and assume rigid SO42δ- with two unconstrained Hδ+ and rigid H2O molecules interacting via revised central force (RSL2) potentials. The potential parameters are determined from both ab initio studies and tests of the potential using the statistical mechanical formalism for binary cluster size distributions. In the potential tests, fixed composition free energy differences, δfkm,m, for [H2O]km[H2SO4]m clusters are plotted versus (km + m)-1/3, and the resulting slope and intercept (in the large cluster regime) are used to extract model dependent binary liquid surface tension and partial vapor pressures at 298 K. The potential parameters are adjusted to obtain approximate agreement with experimental surface tension and partial vapor pressures for k = 1 and 4 (84% and 57% weight percent H2SO4, respectively). The free energy differences for m ≤ 15 are presented, together with internal cluster energy contributions, snapshots of cluster structure, and evidence for onset of the large cluster regime near m = 5. The long-range goals have been to test the free energy difference procedure for studying binary cluster properties and to develop model potentials appropriate for the simulation of small binary clusters at low temperatures characteristic of stratospheric sulfuric acid−water aerosols.

Electron spin resonance aboveTCin layered manganites

We have performed electron spin resonance (ESR) and dc magnetization measurements on single crystals of La{sub 2(1-x)}Sr{sub 1+2x}Mn{sub 2}O{sub 7} up to 800 K with special emphasis on the x=0.4 composition. The ESR linewidth shows behavior similar to that observed in the three-dimensional perovskites and above -500 K can be described by a universal expression {Delta}H{sub pp}(T)=[C/T{sub x}(T)]{Delta}H{sub pp}({sup {infinity}}). The linewidth and the resonance field become anisotropic below -500 K. The anisotropy in the resonance field is proportional to the magnetization M, and we concluded that it is intrinsic to the system. We show that demagnetization effects can explain only part of the anisotropy. The remainder arises from short-range uniaxial terms in the Hamiltonian that are associated with the crystal field and Dzialozhinsky-Moriya interactions. The anisotropy in the linewidth is attributed to the easy-plane ferromagnetic ordering, which also arises from the short-range anisotropy.

Time-Trimming Tricks for Dynamic Simulations: Splitting Force Updates to Reduce Computational Work

Time-Trimming Tricks for Dynamic Simulations: Splitting Force Updates to Reduce Computational Work

Ab initiocoupled-cluster calculations for the fcc and hcp structures of rare-gas solids

In order to gain more insight into factors governing the relative stability of the fcc and hcp structures of the rare-gas solids Ne through Xe, we performed ab initio coupled-cluster calculations for the most important three- and four-body terms in the many-body expansion of the cohesive energy. These terms are combined with empirical two-body potentials derived from dimer data and with a multipole expansion for the long-range three-body terms. In addition, we calculated phonon spectra, in harmonic approximation, for the two structures. Including zero-point energies, our results agree very well with experimental data for the fcc structure. The hypothetical hcp structure, which is lower in energy with two-body potentials, is destabilized by short-range three-body terms and, even more important, by the contribution of zero-point vibration.

Extension of the Wilson model to electrolyte solutions

The Wilson model for non-electrolytes has been extended to model the activity coefficients of electrolytes in aqueous solutions. The excess Gibbs energy of an aqueous electrolyte solution is expressed as a sum of contributions of a long-range and a short-range excess Gibbs energy term. The contribution of the long-range excess Gibbs energy is represented by the Pitzer–Debye–Hückel model. A new expression based on the local composition concept has been developed to account for the contribution of the short-range excess Gibbs energy. The existence of three types of local cells with a central cation, anion or solvent molecule is assumed. Due to the strong like-ion repulsion it is assumed that only counterions can be found in the immediate neighborhood of a central ion. The main difference between this model and the extensions of the NRTL model to electrolytes available in the literature is the assumption that the short-range energy parameter between species in a local cell has an enthalpic rather than Gibbs energy nature. For systems containing only molecular components the new model simplifies to the three-parameter version of the Wilson model. The model has been applied to several single electrolyte systems and it has been shown that it can represent the mean ionic activity coefficients with good accuracy.

The point charge model of nuclear quadrupoles: How and why does it work

The point charge model of nuclear quadrupoles is analyzed beyond the multipole approximation. The effect of the short-range expansion terms is shown to be responsible for the contamination of the calculated electric field gradients. Several regularities observed in earlier numerical calculations are elucidated and their analysis helps to devise proper computational strategies. The use of the point charge nuclear quadrupole model is strongly advocated in approximate relativistic calculations of electric field gradients at heavy nuclei.

A Self-Consistent Charge Density-Functional Based Tight-Binding Method for Predictive Materials Simulations in Physics, Chemistry and Biology

We outline recent developments in quantum mechanical atomistic modelling of complex materials properties that combine the efficiency of semi-empirical quantum-chemistry and tight-binding approaches with the accuracy and transferability of more sophisticated density-functional and post-Hartree-Fock methods with the aim to perform highly predictive materials simulations of technological relevant sizes in physics, chemistry and biology. Following Harris, Foulkes and Haydock, the methods are based on an expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations at a given reference density. While the zeroth order approach is equivalent to a common standard non-self-consistent tight-binding (TB) scheme, at second order by variationally treating the approximate Kohn-Sham energy a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These matrix elements are modified by a Self-Consistent redistribution of Mulliken Charges (SCC). Besides the usual “band-structure” and short-range repulsive terms the final approximate Kohn-Sham energy explicitly includes Coulomb interaction between charge fluctuations. The new SCC-scheme is shown to successfully apply to problems, where defficiencies within the non-SCC standard TB-approach become obvious. These cover defect calculations and surface studies in polar semiconductors (see M. Haugk et al. of this special issue), spectroscopic studies of organic light-emitting thin films, briefly outlined in the present article, and atomistic investigations of biomolecules (see M. Elstner et al. of this special issue).

A simplified force field for describing vibrational protein dynamics over the whole frequency range

The empirical force fields used for protein simulations contain short-ranged terms (chemical bond structure, steric effects, van der Waals interactions) and long-ranged electrostatic contributions. It is well known that both components are important for determining the structure of a protein. We show that the dynamics around a stable equilibrium state can be described by a much simpler midrange force field made up of the chemical bond structure terms plus unspecific harmonic terms with a distance-dependent force constant. A normal mode analysis of such a model can reproduce the experimental density of states as well as a conventional molecular dynamics simulation using a standard force field with long-range electrostatic terms. This finding is consistent with a recent observation that effective Coulomb interactions are short ranged for systems with a sufficiently homogeneous charge distribution.

Transferable ab Initio Intermolecular Potentials. 1. Derivation from Methanol Dimer and Trimer Calculations

An intermolecular potential was derived from ab initio calculations on methanol dimers and trimers. Ninety-four methanol dimer geometries and seventeen trimer geometries have been studied using an interaction-optimized basis set. Because we aimed at transferability, all terms of the model were fitted separately, in order to obtain a potential in which these energy terms have a well-defined physical meaning. Electrostatic interactions were described by atomic multipole moments obtained by fitting to the monomer electrostatic potential. The polarization energy was modeled using atomic dipole polarizabilities. Scaled empirical polarizabilities reproduced the energy nonadditivity in methanol trimers very accurately. The dispersion energy was described by a damped r-6 atom−atom potential, which was fitted to separately calculated dispersion energies. Exchange energy and remaining short-ranged terms were modeled by an exponential repulsion term, including some anisotropic features. This repulsion model was fitt...

Ab initio compact group model potentials for describing environment effects in cluster calculations

A method for the determination of ab initio group model potentials within the Hartree–Fock framework is reported. Following the theory of separability of many electron systems, a new way to incorporate the effect of complete chemical entities by means of polycenter compact model potentials is presented. The interaction between active and frozen electrons is partitioned as a sum of long- and short-range terms. The long-range term is described as the effect of −2e charges placed in the center of the charge of the frozen group molecular orbitals; the short-range one, the exchange and Pauli repulsion, is developed as a spectral representation in a nonorthogonal basis set. An algorithm to solve the problem associated with the rotation of the polycenter model potential is presented and implemented in an all-purpose quantum chemical program. In order to check the method, a group model potential for H2O was obtained and tested. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1145–1152, 1999

Ab initio compact group model potentials for describing environment effects in cluster calculations

A method for the determination of ab initio group model potentials within the Hartree–Fock framework is reported. Following the theory of separability of many electron systems, a new way to incorporate the effect of complete chemical entities by means of polycenter compact model potentials is presented. The interaction between active and frozen electrons is partitioned as a sum of long- and short-range terms. The long-range term is described as the effect of −2e charges placed in the center of the charge of the frozen group molecular orbitals; the short-range one, the exchange and Pauli repulsion, is developed as a spectral representation in a nonorthogonal basis set. An algorithm to solve the problem associated with the rotation of the polycenter model potential is presented and implemented in an all-purpose quantum chemical program. In order to check the method, a group model potential for H2O was obtained and tested. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1145–1152, 1999