Gábor Pálinkás

Car-Parrinello molecular dynamics simulation of the hydrated calcium ion

First principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of [Ca(H2O)n]2+ clusters with n=1–9, and the hydration shell of a calcium ion in a periodically repeated box with 54 water molecules. We find that, while stable highly symmetric Ca–water clusters can be formed with up to eight water molecules, the n=9 cluster dissociates into the last stable [Ca(H2O8]2+ complex. In solution the first hydration shell around the Ca2+ ion contains six water molecules in an octahedral arrangement. The electronic structure of nearest neighbor hydration shell water molecules has been examined with a localized orbital analysis. The average dipole moments of hydration water molecules was found to be increased by about 0.4 Debye relative to that of pure water.

Nonlinear group-contribution models of corrosion inhibition

Abstract The correlation between the molecular structure and inhibitor efficiency (Ei) of triamines was investigated. The group-contribution approach was adapted to account for Langmuir- and Frumki...

Liquid water II. Experimental atom pair-correlation functions of liquid D2O

X-ray, electron and neutron-diffraction data have been used for the separation of the g oo(r), g od(r) and g dd(r) atom-atom pair-correlation functions of liquid D2O. The experimental pair-correlat...

Monte Carlo simulation of liquid acetone with a polarizable molecular model

A Monte Carlo simulation of liquid acetone was performed in the NVT ensemble. The dielectric polarizability of the molecules was taken into account by induced point dipoles. A reference simulation on a system of non-polarizable molecules was also carried out. The induced polarization of the molecules lowered the total energy of the system, while the sum of the pair interaction energies was increased. The structural analysis of the resulting configurations revealed that the average coordination number in the first coordination shell is about 12, and the first one or two nearest neighbours tend to be in an antiparallel orientation relative to the central molecule while the orientation of the rest of the neighbours is random. The predominant interaction in determining the structure of liquid acetone was found to be steric rather than electrostatic. The induced dipole moments of the molecules and the electric field strength at the location of the molecular centres were also examined. The induced dipole moment...

Ethylene glycol dimers in the liquid phase: A study by x-ray and neutron diffraction

Molecular conformations and intermolecular H bonding in liquid ethylene glycol have been studied by neutron diffraction with hydrogen/deuterium isotopic substitution, and by x-ray diffraction measurements at room temperature. For comparison, conformations of molecules and their dimers in the gas phase have also been calculated, based on the density-functional theory. Energies, geometries, and vibrational frequencies of dimers were analyzed. Diffraction data analyzed by the Monte Carlo method resulted in a molecular structure in agreement with the findings from gas-phase electron diffraction experiments and density-functional calculations. Conformers of ethylene glycol molecules in the liquid phase were identified as a gauche OCCO arrangement, stabilized by intramolecular hydrogen bonding, which is in agreement with the results of gas-phase density-functional calculations. Data analysis resulted in an average of three hydrogen-bonded nearest neighbors of molecules in liquid ethylene glycol. In addition, ex...

Molecular dynamics investigation of the inter- and intramolecular motions in liquid methanol and methanol-water mixtures

Molecular dynamics computer simulations were performed at room temperature and selected density on pure liquid methanol and water, and on methanol-water mixtures with methanol mole fractions of 0·1...

Investigation of the uniqueness of the reverse Monte Carlo method: Studies on liquid water

Reverse Monte Carlo (RMC) simulation of liquid water has been performed on the basis of experimental partial pair correlation functions. The resulted configurations were analyzed in various aspects; the hydrogen bond angle distribution, three body correlation and orientational correlation were calculated. The question of the uniqueness of the RMC method was also examined. In order to do this, two conventional computer simulations of liquid water with different potential models were performed, and the resulted pair correlation function sets were fitted by RMC simulations. The resulted configurations were then compared to the original configurations to study how the RMC method can reproduce the original structure. We showed that the configurations produced by the RMC method are not uniquely related to the pair correlation functions even if the interactions in the original system were pairwise additive. Therefore the difference between the original simulated and the RMC configurations can be a measure of the uncertainty of the RMC results on real water. We found that RMC produces less ordered structure than the original one from various aspects. However, the orientational correlations were reproduced rather successfully. The RMC method exaggerates the amount of the close packed patches in the structure, although these patches certainly exist in liquid water.

Liquid water I. Electron scattering

Electron-diffraction measurements for heavy and light water at 5°C are presented and analysed in terms of atom-atom pair-correlation functions and molecular correlation functions.

Hydrogen bonding in liquid methanol, methylamine, and methanethiol studied by molecular-dynamics simulations

Molecular-dynamics computer simulations have been carried out on liquid methanol, methylamine, and methanethiol. The local structure of the liquids was studied based on radial distribution functions and the density projections of the neighboring molecules obtained on the basis of simulated molecular configurations. The extent of hydrogen bonding was investigated by direct analysis of the connectivity of molecules forming hydrogen-bonded clusters in these liquids. By this analysis, the methanol molecules were found to form linear chainlike structures. The local structure of hydrogen-bonded molecules of methylamine proved to be rather space filling due to the great extent of chain branching. Methanethiol molecules also proved to form hydrogen bonds forming small compact clusters. No evidence was found, however, for the clustering of hydrophobic methyl groups in any of the liquids. The quality of simulations was checked by derivation of neutron total and composite radial distribution functions and by comparison of those with available experimental data.

Grand canonical Monte Carlo simulation of liquid argon

A grand canonical Monte Carlo procedure with fixed values of the chemical potential μ, volume V, and temperature T, is described which is suitable to simulate simple fluids with only a minor increase in computer time in comparison with canonical (N,V,T) simulations and considerably faster than (N,p,T) ones. The method is rapidly convergent for rather dense systems with a reduced density of about ρσ3=0.88. The rapid convergence is attained by decreasing the vain attempts in the regime when new particles are added. The chance to find a place for an additional particle is increased by locating the cavities suitable to house a particle with the aid of the Dirichlet–Voronoi polyhedra. As an example, liquid argon is simulated with Lennard‐Jones potentials at T=86.3 K and μ=−73.4 J/mol. The simulated density has been found to be 1.468 g/cm3 which is to be compared with the experimental value of 1.425 g/cm3. The same density was obtained by starting the procedure with both 216 and 250 particles in the simulation ...