Renzo Vallauri

Analysis of the hydrogen-bonded structure of water from ambient to supercritical conditions

The structure of water has been analyzed at eight different thermodynamic states from ambient to supercritical conditions both by molecular dynamics (MD) and Reverse Monte Carlo (RMC) simulation. MD simulations have been carried out with two different potential models, a polarizable potential and one of the most successful nonpolarizable models, i.e., the well known Simple Point Charge potential in its revised version labeled by E (SPC/E). It has been found that, although the polarizable model can reproduce the experimental partial pair correlation functions at the high temperature states better than the nonpolarizable one, it still cannot account for all the features of the measured functions. The experimental partial pair correlation functions have been well reproduced by the RMC simulations at every state point. The resulting structures have been analyzed in detail. It has been found that the tetrahedral orientation of the hydrogen bonded neighbors is already lost at 423k, whereas the hydrogen bonds th...

Parameterizing a polarizable intermolecular potential for water

The parameterization of a polarizable intermolecular potential for water is attempted starting from a simple model of three permanent charges, a Lennard-Jones centre on the oxygen, and a single polarizable site on the centre of mass. It is found that the position of the negative charge is a surprisingly critical variable in parameterizing a successful model. The only model that reproduces 1·0 g cm-3 water properties reasonably well has the negative charge displaced 0·2 A away from the oxygen along the molecular bisector. This model reproduces the correct energy, pressure, diffusion coefficient, and structure of normal water. At supercritical conditions, however, the model, as with non-polarizable models, does not reproduce these properties satisfactorily; the pressure is too high and the diffusion coefficients are too low. In addition, a comparison with published pair distribution functions shows too much OH structure at the lowest density, whereas at the highest density the opposite is true. There are, h...

Analysis of the network topology in liquid water and hydrogen sulphide by computer simulation

The analysis of Voronoi polyhedra for liquid water and hydrogen sulphide, at different temperatures, has been performed by using the molecular configurations generated by computer simulation of the liquids with realistic potential models. Some topological and metric properties of the Voronoi polyhedra have been calculated and their distributions are studied. In addition, the cross correlation between pairs of metric quantities are also investigated. The latter correlations are found to be more relevant for a clear distinction between the two systems examined here. In particular, the cross correlation between the potential energy of a molecule and the volume of the corresponding Voronoi polyhedron makes it clear that the interpretation of the anomalous physical properties of water in terms of local volume has to be revised.

Liquid–vapor and liquid–liquid phase equilibria of the Brodholt–Sampoli–Vallauri polarizable water model

Liquid-vapor and liquid-liquid phase equilibria of the polarizable Brodholt-Sampoli-Vallauri water model have been investigated by Gibbs ensemble Monte Carlo computer simulations. The coexisting liquid and vapor densities and energy of vaporization of the model is found to be in a reasonable agreement with experimental data in the entire temperature range of liquid-vapor coexistence. The critical temperature and density of the model are found to be 615 K and 0.278 gcm(3), respectively, close to the experimental values of 647.1 K and 0.322 gcm(3). In the supercooled state two distinct liquid-liquid coexistence regions are observed. The existence of liquid-liquid phase separation of a polarizable water model is demonstrated for the first time.

Molecular dynamics results for stretched water

Detailed computer simulation results of several static and dynamical properties of water, obtained by using a realistic potential model proposed by Jorgensen et al., in the supercooled region, at densities well below the coexistence curve, are reported. We have analyzed the structural properties by evaluating the volume distributions of Voronoi polyhedra as well as angular and radial distributions of molecular clusters. In particular, the homogeneity of the system has carefully been checked. The investigated dynamical properties mainly concern the density and temperature dependence of the diffusion coefficient. The results are compared both with previous simulations, performed with different models, and with experimental findings. Some differences stress the fact that the conclusions drawn on the physical process underlying density and temperature behavior, can strongly be influenced by the use of different potential models.

Computer simulations of liquid HF by a newly developed polarizable potential model

A new polarizable potential model of liquid HF is developed. The model is tested by comparing simulated thermodynamic and structural properties of liquid HF with experimental and ab initio molecular dynamics results. Properties of the isolated HF dimer obtained with this model are also compared with both ab initio and experimental data. It turns out that this model can describe the thermodynamic properties of liquid HF with a good accuracy in the entire temperature range of the liquid state at atmospheric pressure, and also give the energy and geometry of the isolated HF dimer in a good agreement with experimental and ab initio results. The obtained pair correlation functions of liquid HF agree also reasonably well with experimental and ab initio molecular dynamics findings, although the position of the peaks describing hydrogen bonding appears to be at 0.1–0.3 A higher distances. Such a deviation is opposite to what has been observed with our previous nonpolarizable model, which resulted in peak position...

Analysis of the velocity autocorrelation function of water

The centre-of-mass velocity autocorrelation function of liquid water under room conditions is examined through a mode-coupling approach developed to study monatomic liquids. The wavevector-dependent current correlations relevant to the theory are evaluated by computer simulations using a recently developed interaction potential, which accounts for the polarizability of the molecule. The theory is found to reproduce all the relevant dynamical features of water. Moreover, the analysis in terms of longitudinal and transverse contributions clarifies the origin of some peculiarities in the dynamics of this single-molecule correlation. A discussion of the corresponding spectra is also given.

Hydrogen bonded clusters in the liquid phase: I. Analysis of the velocity correlation function of water triplets

We present an analysis of hydrogen bonded triplet clusters of water in terms of velocity projections on locally defined molecular coordinates. This makes it possible to perform an approximate normal mode analysis and to establish the relative motions of hydrogen bonded molecules. We find that the interpretation of the velocity autocorrelation function (VACF) of water in terms of stretching and bending motions of hydrogen bonded neighbours is an oversimplification due to cooperative effects in the hydrogen bonded network. Especially, for the peak at we find that it is not only due to O-O-O bending motions in hydrogen bonded groups but also to librational motions of the whole cluster.

Computer simulation study of liquid HF with a new effective pair potential model

A new three-site effective pair potential has been developed to reproduce the properties of liquid hydrogen fluoride. Computer simulations have been performed with the new model at two thermodynamic state points. Thermodynamic properties and pair correlation functions are presented and compared with experimental data. The partial pair correlation functions obtained are compared also with the results of a reverse Monte Carlo simulation using neutron diffraction data. The newly developed potential model is found to be more realistic than the previous models.

Interparticle velocity correlations in simple liquids

The momentum transfer of a test particle to a cluster of atoms initially in the first shell of neighbors is investigated at liquid densities by molecular dynamics methods. The comparison of the results obtained for Lennard‐Jones and soft spheres models is discussed. In the short time regime the interpretation of the data is shown to parallel that of the single particle velocity autocorrelation function. At longer times, a simple physical model can account for the decay of the cross correlation in terms of the increasing number of atoms involved in the process.