Tamás Kristóf

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.

Simulation and experimental study of intercalation of urea in kaolinite

Experimental measurements and molecular simulations were used to describe the characteristics of the kaolinite/urea intercalation compound. The intercalation compound was synthesized by a mechanochemical method and examined by X-ray diffraction and thermogravimetry. Additionally, a series of NpT (constant particle number-pressure-temperature) simulations was performed to identify thermodynamically stable basal spacings. From the simulations the most probable molecular orientations were determined for single and double layered arrangements of urea molecules that develop between the layers of kaolinite.

Current and selectivity in a model sodium channel under physiological conditions: Dynamic Monte Carlo simulations.

A reduced model of a sodium channel is analyzed using Dynamic Monte Carlo simulations. These include the first simulations of ionic current under approximately physiological ionic conditions through a model sodium channel and an analysis of how mutations of the sodium channels DEKA selectivity filter motif transform the channel from being Na(+) selective to being Ca(2+) selective. Even though the model of the pore, amino acids, and permeant ions is simplified, the model reproduces the fundamental properties of a sodium channel (e.g., 10 to 1 Na(+) over K(+) selectivity, Ca(2+) exclusion, and Ca(2+) selectivity after several point mutations). In this model pore, ions move through the pore one at a time by simple diffusion and Na(+) versus K(+) selectivity is due to both the larger K(+) not fitting well into the selectivity filter that contains amino acid terminal groups and K(+) moving more slowly (compared to Na(+)) when it is in the selectivity filter.

Alternative implementations of the Gibbs ensemble Monte Carlo calculation

Abstract Two new versions (NpH and μVL) of the Gibbs ensemble Monte Carlo simulation for pure fluids are suggested. These procedures allow the change of heat between the correlated subsystems. In the determination of the vapour-liquid coexistence, the capability of the new algorithms is similar to that of the original (NVT) method, however, in the determination of the vapour pressure curves, the NpH version has some advantage over the others.

Simulation-assisted evidence for the existence of two stable kaolinite/potassium acetate intercalate complexes.

Recent molecular simulation findings with several kaolinite intercalate complexes raised the question of the existence of more than one stable state, which has not been confirmed by experimental observations yet. Kaolinite/potassium acetate intercalate complexes were synthesized and examined by X-ray diffraction, and a molecular simulation study was performed for the system. Consistent with the suggestion from the simulations, an additional stable basal spacing was found experimentally at d(001)=1.168nm besides the well-known one at d(001)=1.403nm.

A Gibbs ensemble Monte Carlo study of phase coexistence in the solvent primitive model

The phase coexistence behavior of the solvent primitive model (SPM) is studied by constant pressure and temperature Gibbs ensemble Monte Carlo simulations. In the SPM, the ions are modeled with charged hard spheres, while the solvent molecules are represented by neutral hard spheres. Fluid–fluid phase separation into a salt poor and salt rich phase is found. At constant pressure, the critical temperature increases with respect to the critical temperature of the primitive model (PM) where no hard spheres are present. At constant temperature and for low pressures, the phase separation of the SPM transforms into the phase separation of the PM. For high pressures, it remains an open question whether there is an upper critical immiscibility pressure or whether a solid–fluid phase transition occurs first.

Prediction of adsorption equilibria of water–methanol mixtures in zeolite NaA by molecular simulation

Predictions for the adsorption of mixtures of water and methanol in zeolite NaA are reported. The pressure dependence of the adsorption properties such as equilibrium amounts of adsorption and isosteric heats of adsorption are calculated at 378 K by molecular simulations using effective pair potential models. These data are also determined for the adsorption from liquid mixtures. The models predict selectivity inversion in the investigated range of pressure. The change in adsorption ratios can partly be explained by the structural characteristics of the system.

Vapour-liquid equilibrium of the charged Yukawa fluid from Gibbs ensemble Monte Carlo simulations and the mean spherical approximation

A Gibbs ensemble Monte Carlo (GEMC) study supplemented by theoretical calculations using the mean spherical approximation (MSA) is reported for the charged Yukawa system. In this system the particles interact via an attractive Yukawa potential and a Coulomb potential. When the Coulomb potential is weak compared to the Yukawa attraction the results obtained for the vapour-liquid equilibrium from the GEMC and the MSA are very similar. On increasing the charge, the system becomes similar to the primitive model of electrolytes, and at high charges similar phenomena, such as ion association, are observed. At these charges, the MSA yields poorer results. Using both methods, an increase of the charge results in an increase of the critical temperature but a decrease of the critical density.

Dynamic Monte Carlo simulation in mixtures

The dynamic Monte Carlo technique is a widely used simulation tool but the parameters of the calculation have to be tuned to reflect the same dynamics as the corresponding molecular dynamics simulation. As the direct calibration of the dynamic Monte Carlo with molecular dynamics is a laborious task, we propose a new method that allows the standard dynamic Monte Carlo to realize the correct time proportionality in many-component systems without the need of corresponding molecular dynamics calculation. The method has been tested in various systems and the dynamic Monte Carlo results obtained by the proposed method were found to be in good agreement with the results of the control molecular dynamics simulations.

A new simulation method for the determination of phase equilibria in mixtures in the grand canonical ensemble

A grand canonical Monte Carlo (GCMC) simulation method is presented for the determination of the phase equilibria of mixtures. The coexistence is derived by expanding the pressure into a Taylor series as a function of the temperature and the chemical potentials that are the independent intensive variables of the grand canonical ensemble. The coefficients of the Taylor series can be calculated from ensemble averages and fluctuation formulae that are obtained from GCMC simulations in both phases. The method is able to produce the equilibrium data in a certain domain of the (T, p) plane from two GCMC simulations. The vapour-liquid equilibrium results obtained for a Lennard-Jones mixture agree well with the corresponding Gibbs ensemble Monte Carlo data.