Polarizable Point Charge Model Research Articles

Fugacity Coefficients of Saturated Water from Molecular Simulation

We apply isothermal−isobaric Monte Carlo molecular simulation to measure the vapor-phase fugacity coefficients and equation of state of three fixed-charge models of water: SPC/E, MSPC/E, and TIP3P. State conditions correspond to the experimental vapor−liquid saturation pressures for temperatures between 292.65 and 386.85 K. The results are compared to the experimental data for water vapor. We also examine properties as given by the application of the virial equation of state, using values of the second virial coefficients computed by us or that have been previously reported (in some cases up to third virial coefficients) for the models simulated here. The models show significant deviation from ideal-gas behavior, considerably more than seen experimentally, and a second-order virial treatment does not fully characterize the behavior. This deviation from experiment is consistent with the expected failings of fixed-charge models developed for liquid phases, when applied to the vapor. A virial-equation treatment of a fluctuating-charge model (TIP4P/fq) and the polarizable point charge model (PPC) indicates that the vapor-phase properties of these models will be much more in line with experimental fugacities and densities along the coexistence line.

Effects of the polarizability and water density constraint on the structure of water near charged surfaces: Molecular dynamics simulations

To understand how the inclusion of explicit polarizability into water interaction potential changes the structure of water next to a charged metallic surface we compared the results from the simulations using polarizable point charge model and simple point charge model. To study the effect of density constraints we also performed simulations of water next to hydrophobic walls and metal walls. In these simulations the water density was not predetermined, but regulated itself during the run.

Nuclear magnetic resonance quadrupolar parameters and short range orderin disordered ionic fluorides

Information on both radial and angular atomic coordinations is provided by thequadrupole interaction between the quadrupole moment of the atomic nuclei andthe electric field gradients (EFGs) originating from the distribution ofelectric charges around the nuclei. 69Ga and 71Ga arequadrupolar nuclei active in NMR. Their spectra are recorded in amorphousGaF3 which may be considered as a model compound for disorderedfluorides with fluorine corner sharing octahedra. An unique and continuousquadrupolar parameter Czjzek distribution allows us to simulate theexperimental spectra. The measured chemical shifts indicate that the Ga3+ ions are at the centre of (GaF6)3- octahedra in theamorphous GaF3 phase.A polarizable point charge model is used to explain NMR quadrupolar parameterswhich are related to the electric field gradients at the Ga site. Latticesummations are performed in the direct space over spherical volumes. Atomicposition sets generated by molecular dynamics are shown to give quadrupolarparameter distributions which look like Czjzek ones whatever the EFGcalculation approximations. Provided the polarizabilities are adjusted, theyallow us to reconstruct the experimental NMR spectra which correspond in anycase to slightly distorted (GaF6)3- octahedra. The presentapproach may be applied to any ionic disordered compound which containsquadrupolar nuclei and used to quantify short range order around such nuclei.

Phase coexistence properties for the polarizable point charge model of water and the effects of applied electric field

Extensive efforts are currently devoted to the development of water models for computer simulations that explicitly incorporate molecular polarizability. Liquid–vapor coexistence properties for the polarizable point charge (PPC) model of water are examined in this contribution with the aid of molecular dynamics calculations. An accurate analytical equation of state for the high-temperature states of the PPC model is presented, including its critical region. The liquid–vapor coexistence curve and the critical point parameters for the PPC water are extracted from its equation of state. The critical temperature, pressure, and density for the PPC water are found to be within 10% of their experimental values and its critical compression factor is within 2%. The effect of applying an external electric field to the system on the coexistence properties is investigated. A shift of the critical point of water to higher temperature and pressure is observed, a result similar to that of the introduction of an electrolyte to water. A large applied electric field is capable of inducing phase separation of a subcritical fluid on the time scale of a typical simulation run.

Polarizable point-charge model for water: Results under normal and extreme conditions

Molecular dynamics simulations of liquid water under normal and extreme conditions are performed using the polarizable point-charge (PPC) model. This efficient three-site model explicitly incorporates results from ab initio studies of the water molecule in applied electric fields. The structural, thermodynamic, and dielectric properties, and the self-diffusion coefficient are examined at a number of temperatures ranging from 263 to 573 K. These simulation results are compared with available experimental data along the liquid–vapor coexistence line; the agreement is very good for all properties studied. The temperature of maximum density for the PPC model is found to coincide with the experimentally observed value of 277 K. The spatial coordination of water molecules in the liquid and the anisotropy of the self-diffusion tensor are analyzed at various state points. Increased directional anisotropy in the local translational diffusion, suggestive of prenucleation phenomena, can be observed at T=263 K. Above T=473 K the local translational anisotropy becomes rather insensitive to temperature variation indicating a weakening of the correlations between water molecules. Rototranslational dynamics and nonlinear polarization effects arising in polarizable models for water are discussed along with their phenomenological implications. The dimer properties for the PPC potential are also reported.

Static Model and Order–Disorder Model in RbCaF3. Electric Field Gradient Calculations in the Quadratic Phase

The polarizable point charge model is used to calculate the EFG tensor at the rubidium site in the tetragonal phase of RbCaF3 using two structural models: the static model and the order–disorder type model for comparison with the experimental 87Rb NMR results. The static model provides the experimental number of NMR lines, but the numerical values of the resonances arc different. The order–disorder model predicts a different NMR spectrum. This discrepancy, which does not disqualify the model, could be due to the jump frequency of fluorine ions between the different equivalent potential wells, which could be much higher than the NMR measuring frequency, leading to an experimental “averaged” NMR spectrum. Le modèle de charges ponctuelles polarisables a été utilisé pour calculer le tenscur GCE au site rubidium dans la phase quadratique de RbCaF3 en utilisant deux modèles structuraux: modèle statique et modéle de type ordre- désordre. Les résultats théoriques sont comparés aux résultats expérimentaux de RMN de 87Rb. Le modéle statique prévoit le nombre de raies observées expérimentalement, mais les valeurs numériques de résonance sont différentes. Le modèle ordre-desordre prévoit un spectre RMN différent du spectre expérimental. Cette différence, qui n'élimine pas la validité de ce modéle, est interprétée à partir de la fréquence de saut des ions fluor entre les différents puits de potcnticl équiprobables, qui serait très supe̊rieure à la fréquence de mesure en RMN ct conduirait expérimentalement à un spectre RMN “moyen”.

Electric field gradient in LiMnFeF 6 compounds. polarizable point charge calculations and Mössbauer data

The polarizable point charge model is used to explain Mössbauer data on the electric field gradient (EFG) at the iron site in α- and β-LiMnFeF 6. Lattice summations are performed in direct space. The fluorine polarizability which fits the data is in reasonable agreement with the expected value (α F∼ 1.2 A ̊ 3) The dipolar contribution is found to be important. In both cases the main EFG component is positive. In β-LiMnFeF 6, according to the structure, the EFG tensor is axial with respect to the c axis; in α-LiMnFeF 6, the asymmetry parameter η is large (≅0.5) and the OZ axis is along the a axis.

Off-center lithium ions in KTaO3

We present a polarizable point-charge model for the KTa${\mathrm{O}}_{3}$ lattice that permits the calculation of the off-center displacement $d$ of Li ions that substitute for K. We find $d=1.35$ \AA{} in good agreement with a value $d=1.26$ \AA{} derived from NMR measurements. The permanent dipole created by the off-center Li ion is found to be screened by the dipoles induced in the lattice: The "screening volume" is strongly anisotropic, reminiscent of the correlation volume for polarization fluctuations. The dielectric susceptibility of the Li-dipole system is considerably enhanced by local-field effects.

Mapping of second-nearest-neighbor fluoride ions of orthorhombic Gd 3+-Ag + complexes in CaF 2

The ENDOR technique is applied to determine the positions of 24 second-nearest-neighbor F − ions around an orthorhombic Gd 3+-Ag + complex in CaF 2 crystals. Experimental ENDOR data of the second-nearest-neighbor F − ions are analyzed by using the usual spin Hamiltonian and a least-squares fitting method. The best fits of the experimental results give superhyperfine (shf) constants and the F − directions ( K, L, M) with respect to the Gd 3+ ion, from which the distance between the second-nearest-neighbor F − ion and the Gd 3+ ion is determined by assuming that the hyperfine interaction is due to the classical dipole-dipole interaction. The displacements of the F − ions are estimated and compared with the theoretical values calculated by Bijvank and den Hartog on the basis of a polarizable point charge model.

Electric field gradients in AFe IIIF 4 structures: Calculations for the polarizable point charge model and Mössbauer data

The polarizable point charge model is used to explain Mossbauer data on the EFG at the iron site in AFe F 4 ( A = K, Cs, Rb, NH 4). Lattice summations are performed in direct space. The resolution of the self consistent equations involving the electric field is obtained through an iterative method. The major contribution to the EFG is that of “terminal” (non-bridging) fluorines. The fluorine polarizabilities which fit the data are in reasonable agreement with expected values (α F ≅ 0.9 Å 3).