Simple Pair Potentials Research Articles

Shear viscosity and dielectric constant in aqueous isopropanol and aqueous acetonitrile

The importance of aqueous mixtures as solvents has motivated the investigation of the usefulness of classical molecular dynamics simulation, using simple effective pair potentials and the Lorentz-Berthelot combining rules, to predict mixture viscosities and dielectric constants. In the acetonitrile-water system, near quantitative agreement with experiment is found; however, the calculated shear viscosity in the 2-propanol-water system is underpredicted by more than a factor of two. Moreover, the results do not appear to be strongly dependent on the mixture combining rules, as a 10% decrease in the unlike OO Lennard-Jones size parameter for this system had no discernable effect on the calculated viscosity. In both systems, the dielectric constant predictions were in qualitative accord with experiment, although systematically underpredicted as in the pure component models.

A simple effective pair potential for the molecular simulation of the thermodynamic properties of ammonia

A new optimized effective pair potential model is proposed, which is appropriate for the prediction of thermodynamic properties of fluid ammonia including vapour—liquid coexistence data. The phase behaviour is determined using a recently developed version of the Gibbs ensemble Monte Carlo method. Furthermore, liquid structure characteristics, the dielectric constant and supercritical properties are determined by Monte Carlo simulations in the isothermal—isobaric ensemble. The second virial coefficient of the pair potential model is calculated over a broad range of temperature. All properties are compared with experimental data or results of a multi-parameter equation of state for ammonia. The new model is found to yield coexistence properties and second virial coefficients in good agreement with experimental data and the results of the equation of state, respectively.

A simple effective pair potential for the molecular simulation of the thermodynamic properties of ammonia

A simple effective pair potential for the molecular simulation of the thermodynamic properties of ammonia

Structure of molten MCl3 systems from a polarizable ion simulation model

Computer simulations of a range of molten salts of stoichiometry MX3 using a polarizable, formal charge ionic interaction model are described. The systems studied — LaCl3, TbCl3, and YCl3 — span a range of cation sizes and the interaction model is a “generic” one, in the sense that the cation size is the only parameter in the interaction potential which distinguishes one system from another. The liquid structures predicted from the simulations are compared with recently obtained neutron diffraction data. Excellent agreement is found, except that the first coordination shell seems to be too tightly bound in the computer simulations. The cation in LaCl3 is found to be 7–8 fold coordinate, and the coordination number drops to 6 for the smallest cation considered (Y3+), so that the coordination number in these systems does not change substantially on melting, in contrast to earlier reports. The polarization effects promote a significant degree of edge-sharing between these coordination polyhedra relative to predictions of a simple ionic pair potential. Associated with these changes is a shift to smaller scattering vectors of the prepeak in the structure factor and an increase in the fluidity.

Accurate Estimates of Infinite-Dilution Chemical Potentials of Small Hydrocarbons in Water via Molecular Dynamics Simulation

The infinite dilution chemical potentials of methane, ethane, propane, and benzene in water are precisely estimated using the single charging integral method [Chialvo, J. Chem. Phys. 1990, 92, 673; Chialvo, J. Phys. Chem. 1991, 95, 6683] in conjunction with molecular dynamics (MD) simulation using simple pair potentials. The method requires the calculation of ensemble averages of derivatives of the potential energy with respect to the model parameters. The derivatives take the form of pairwise virials and energies which are easily obtained with minor modifications to an existing MD code. The present approach is found to be both stable and precise and allows individual contributions of the model parameters to the free energies to be determined. The dependence of the results for the benzene system on the size of the system is investigated, as is the sensitivity of all of the results to the Lorentz−Berthelot combining rules. The importance of including long-ranged corrections in the results is highlighted. The computed chemical potentials for the united atom hydrocarbon models agree with experiment to within 0.7 kJ/mol.

Pb/Cu (100) surface superstructures: Monte Carlo and molecular dynamics simulations

Monte Carlo simulations with simple pair potentials of the Lennard-Jones type enable us to show the stability of the three experimentally known superstructures of Pb/Cu (100) at different lead submonolayer coverages: c(4 × 4)atθ = 3/8,c(2 × 2)atθ = 0.5 and c(5√2 × √2)R45° at θ = 0.6. In addition, numerous details of these superstructures, including interatomic distances, surface alloying, corrugation and weak modulation are obtained numerically in quantitative and qualitative accord with the experimentally observed and measured data. By molecular dynamics the melting of these structures is studied from the temperature dependence of the Pb-atom average energy and diffusion coefficient, with evidence for a first-order transition for every superstructure. The dispersion of surface phonons is also derived.

The Flat Phase of Crystalline Membranes

We present the results of a high-statistics Monte Carlo simulation of a phantom crystalline (fixed-connectivity) membrane with free boundary. We verify the existence of a flat phase by examining lattices of size up to $128^2$. The Hamiltonian of the model is the sum of a simple spring pair potential, with no hard-core repulsion, and bending energy. The only free parameter is the the bending rigidity $\kappa$. In-plane elastic constants are not explicitly introduced. We obtain the remarkable result that this simple model dynamically generates the elastic constants required to stabilise the flat phase. We present measurements of the size (Flory) exponent $\nu$ and the roughness exponent $\zeta$. We also determine the critical exponents $\eta$ and $\eta_u$ describing the scale dependence of the bending rigidity ($\kappa(q) \sim q^{-\eta}$) and the induced elastic constants ($\lambda(q) \sim \mu(q) \sim q^{\eta_u}$). At bending rigidity $\kappa = 1.1$, we find $\nu = 0.95(5)$ (Hausdorff dimension $d_H = 2/\nu = 2.1(1)$), $\zeta = 0.64(2)$ and $\eta_u = 0.50(1)$. These results are consistent with the scaling relation $\zeta = (2+\eta_u)/4$. The additional scaling relation $\eta = 2(1-\zeta)$ implies $\eta = 0.72(4)$. A direct measurement of $\eta$ from the power-law decay of the normal-normal correlation function yields $\eta \approx 0.6$ on the $128^2$ lattice.

Molecular-dynamics study of the melting of hexagonal and square lattices in two dimensions.

Simple pair potentials are discussed which give hexagonal- and square-lattice ground states in two dimensions. Molecular-dynamics calculations for these potentials are used to study transitions between the two lattices and melting. The melting temperature is found to be a minimum near the point where the barrier between the two structures, as a function of lattice strain, is also a minimum.

Quadrupole Polarization in Simulations of Ionic Systems: Application to AgCl

A method which allows for the effects of dipole and quadrupole polarizations on the interparticle interactions in molecular dynamics simulations is described. Multipole induction by both Coulombic and short-range interactions is included. The validity of a description of the interactions in condensed ionic systems with pair potentials supplemented with a description of polarization effects is assessed, and the implications for reliably parameterizing such a model are discussed. The role of cation quadrupole polarization is explored in simulations of solid and liquid AgCl. The effect is to substantially improve the agreement with experiment of properties such as phonon dispersion and liquid structure relative to calculations with a simple pair potential.

An improved pair potential to recognize native protein folds

We present a novel method to improve a simple pair potential of mean force, derived from experimentally determined protein structures, in such a way that it recognizes native protein folds with high reliability. This improvement is based on the use of mutation data matrices to overcome difficulties arising from the poor statistics of small sample sizes. A set of 167 protein chains taken from the Brookhaven Protein Structure Data Base, selected from high-resolution structures and avoiding homologous proteins, is used for generation of the potential set. The potential describes interresidue pair energies depending on distance and sequential separation, and is calculated using the Boltzmann equation. Its performance is evaluated by jackknife tests that try to identify the native fold for a given sequence among a large number of possible threadings on all structures in the set without allowing for gaps. Up to 94% of the protein chains are correctly assigned to their native folds, so that all proper single-chain domains are recognized.

Transferable total-energy parametrizations for metals: Applications to elastic-constant determination

In this work we present a method of fitting augmented-plane-wave energy bands and total-energy results for the fcc and bcc structures to a nonorthogonal tight-binding Hamiltonian from which the sum of the eigenvalues is derived, and to a simple pair potential that is designed to account for all the other terms of the total-energy expression. We have applied this method to calculate the elastic constants of the metals Rh, Pd, Ta, Ir, and Au, avoiding the expensive first-principles calculations of the distorted structures. We obtained very good agreement with both first-principles local-density-functional-theory calculations and experiment.

Theoretical study of water trimer

Geometry optimizations, rearrangement mechanisms, reaction pathways, spectral intensities, and tunneling splittings are reported for the water trimer at various levels of theory. Three of the four pathways which exist for a simple empirical rigid-molecule effective pair potential appear to be quite realistic, as evidenced by ab initio calculations of increasing quality up to DZP basis sets with correlation energy corrections from second-order MOller-Plesset theory. The spectroscopic consequences of the nonrigidity implied by the various rerrangement mechanisms are also considered; the pathways permit the interconversion within two subsets of the 96 permutationally distinct (H 2 O) 3 clusters that arise when only O-H hydrogen bonds, as opposed to chemical bonds, may break

Phenomenological many-body potentials from the interstitial electron model. I. Dynamic properties of metals

Inspired by the ab initio generalized-valence-bond calculations of small metal clusters, we propose a phenomenological many-body interaction model, the interstitial electron model (IEM), for interactions of ions and electrons in metals. In this model, the valence electrons are treated as classical particles situated at the crystal lattice interstitial positions. Simple pair potentials are used for ions and interstitial electrons, allowing the inhomogeneity and anisotropy of electron density distributions to be taken into account phenomenologically. To test the efficacy and applicability of this approach, the IEM is applied to lattice dynamics in fcc metals: Cu, Ni, Ag, Au, Pd, Pt, Al, Ca, Sr, and γ-Fe. The phonon dispersion relations, densities of states, and Debye temperature are calculated and found to be in good agreement with experiments. Extension of the IEM to the construction of a new many-body potential in metals and alloys is discussed.

Oxygen-oxygen pair potentials and the origin of nonstoichiometry in yttrium barium copper oxide YBa2Cu3O7-.delta.

The use of different types of pair potentials is explored in an effort to mimic the energetics of the interplanar oxygen atoms in YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]]. It is shown how the Sterne-Wille V[sub i] parameters obtained from the Connolly-Williams formalism are dependent upon the identity of the structures included in the process. Simple two-atom pair potentials derived from calculations where there is no choice of structure or stoichiometry are used to view the oxygen motion. Even within this scheme there are three types of pair potentials. Two are appropriate for the study of processes where atoms or ions are lost from the structure. The third pair potential which uses two merged densities of states is appropriate for the study of ordering processes in materials of fixed stoichiometry. The d count variation of these three parameters leads to useful insights into the behavior of the oxygen atoms in the interplanar region of YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]]. Plane-to-chain electron transfer in this compound (and thus an increase in the d count of the chain oxygen atoms above 8) is a vital ingredient in leading to repulsive oxygen-oxygen potentials which lead to oxygen loss and hence nonstoichiometry. 17 refs., 12 figs., 2more » tabs.« less

The trans-stilbene-Ar van der Waals complex. Vibrationally averaged substitution structure in its S 0 and S 1 electronic states

Argon atom substitution coordinates, accurate to ±0.02 Å, in the S 0 and S 1 states of the trans-stilbene-Ar van der Waals complex have been determined by comparing the inertial constants obtained from fits of two bands in the rotationally resolved electronic spectrum of the complex with those obtained from fits of the corresponding spectrum of the bare molecule (Champagne et al., J. Phys. Chem. 94 (1990) 6). The argon atom is found to be localized above (or below) the plane of one of the aromatic rings by ∼ 3 Å in both electronic states but is displaced off the ring center by more than ∼ 1 Å in both transverse directions. This geometry is not the one predicted by simple atom-atom pair potentials. Model calculations suggest that this is a consequence of the vibrational averaging that occurs on the S 0 and S 1 surfaces, and that these surfaces are significantly influenced bythe anisotropy of the v

Density-dependent potentials for simulations

The authors have obtained a simple analytic pair potential and volume potential for simulations of simple metals in which the density dependence of the parameters is given explicitly. The potentials are based on a first-principles nonlocal pseudopotential theory with a correction term in the volume potential to allow for the effects of the density dependence of terms in the total energy beyond second order in perturbation theory. This correction term gives the pressure required to obtain the measured equilibrium lattice constant. In this paper the authors present a potential for Li and show that it gives the FCC structure at low temperatures, and gives good agreement with measured phonon dispersion curves and elastic constants.

Ionic dynamics in computer simulated molten LiNO3. III. Effect of the potential well on the translational and reorientational motions

In order to study the effect of the potential well on the single ion dynamics and the charge transport in molten LiNO3, a molecular dynamics simulation, which included all degrees of freedom of vibrating nitrate ions, was carried out. Simple Coulomb pair potential with Born-type repulsion was adopted as a standard, and a potential well of varying depth between Li+ and O of NO−3 was added to the standard potential. As the well depth increased, both the translational and orientational diffusion were found to slow down, leading to solid like dynamics. The potential dependence of the ionic dynamics was studied from the translational, orientational, and current correlation functions. Results of the simulation were compared with the experimental diffusion constants, static conductivity, and rotational behavior revealed by Raman spectroscopy. Using the relation between the infrared absorption spectrum and the autocorrelation function of the dipole moment (or the current) of the system, we carried out the first simulation of translational band shapes in an ionic melt, and demonstrated how the absorption spectrum was related to the velocity correlation functions of cations and anions. The infrared and Raman spectroscopy were found to give sensitive criteria for finding a good potential of an ionic liquid. The lifetime of local structures surrounding nitrate ions, which were characterized by the coordination number, was found to increase remarkably as the well depth increased. The coupling between the environmental structure and the translational and rotational behavior of a nitrate ion was examined through selectively sampled correlation functions.

Finding saddle points for clusters

The Cerjan–Miller eigenvector-following method is highly successful in finding saddle points for a variety of clusters whose potential energy surfaces are well described using simple analytical pair potentials. Examples are given for argon clusters, including Ar33 and Ar55, potassium chloride clusters and clusters of ions held in an ‘‘ion trap.’’ The method reveals a number of important patterns and opens the way to a more detailed understanding of the relation between energetics, reactivity, and potential energy surfaces for these and other systems.

A neutron diffraction and molecular dynamics investigation of the structure of vitreous beryllium fluoride

A combined neutron diffraction and molecular dynamics study is reported for vitreous beryllium fluoride. The structure of vitreous BeF 2 is remarkably similar to that of vitreous silica and may be envusaged as a random network of corner-sharing BeF 4 tetrahedra, with a BeF bond length of (1.553 ± 0.005) Å. In contrast to the predictions of the molecular dynamics simulation using simple pair potentials, the BeF 4 tetrahedra in the glass are extremely regular and network disorder arises from a wide distribution of BeFBe and bond torsion angles. It is this last aspect of the glass structure which eliminates structural models based on the crystalline BeF 2 polymorphs.

The equilibrium structure of highly polar molecular liquids

Neutron diffraction measurements have been performed at three temperatures covering the liquid range of SO2. Refined molecular structure parameters have been derived, and new results concerning the intermolecular single particle pair correlation functions are presented. The short-range liquid structure demonstrates the presence of strong oriental correlations which are difficult to explain in terms of simple pair potentials.