E. Guàrdia

Molecular dynamics simulation of liquid water along the coexistence curve: Hydrogen bonds and vibrational spectra

Structure and dynamic properties of liquid water at temperatures between 298 and 523 K and densities between 0.75 and 1.20 g/cm3 have been investigated by molecular dynamics simulation. A flexible simple point charge potential has been asssumed for interactions. The hydrogen bonding structure in the different simulated states as well as the influence of the hydrogen bonds on the dynamic properties (self‐diffusion coefficients, vibrational spectra) is discussed. Special attention is paid to the intermolecular vibrational spectrum (10–400 cm−1). It has been corroborated that the band around 200 cm−1 can be attributed to intermolecular O–O stretching vibrations of pairs of H‐bonded bounded molecules. On the contrary, molecular dynamics results indicate that the band close to 50 cm−1 is independent of the existence of hydrogen bonds but depends on the density and temperature of the system. It is suggested that it is simply associated with vibrations of molecules in the cage formed by their neighbors. Shifts of librational and stretching bands as a function of the thermodynamic state are highly correlated with changes in the percentage of hydrogen bonded molecules.

Na+–Na+ and Cl−–Cl− ion pairs in water: Mean force potentials by constrained molecular dynamics

Molecular dynamics simulations of Na++Na+, Na++Cl−, and Cl−+Cl− ions in dilute aqueous solution were carried out using a flexible single point charge (SPC) model for water. The resulting structural and dynamic properties are compared with experimental data and other computer simulation results. The potentials of mean force [W(r)] between the like ions were determined from constrained molecular dynamics simulations. The resulting W(r) for the Na+–Na+ ion pair is in qualitative agreement with other computer simulation findings, whereas the discrepancies are important in the case of the Cl−–Cl− ion pair. Our Cl−–Cl− mean force potential shows a moderate minimum which does not involve the unexpected strong attraction between chloride ions at short distances as predicted in earlier papers. The solvent structure around the ion pairs for separations corresponding to the maxima and minima of the W(r)’s is analyzed.

Hydrogen bonding in liquid alcohols: a computer simulation study

Abstract A series of molecular dynamics simulations has been performed to investigate hydrogen bonding in liquid alcohols. The systems considered have been methanol, ethanol, ethylene glycol and glycerol at 298 K. The hydrogen bonding statistics as well as the mean lifetime of the hydrogen bonds are analyzed. The results are compared with those corresponding to liquid water.

Dielectric properties and infrared spectra of liquid water: Influence of the dynamic cross correlations

Molecular dynamics simulation is applied to the study of the infrared spectra associated to translational, librational, and vibrational motions as well as to dielectric relaxation of liquid water. Flexible simple point charge (SPC) potentials are used in the simulations. Both dielectric and infrared spectra are calculated and compared with available experimental data. The effects of the dynamic cross correlations are analyzed by comparing the results for the time correlation functions of collective and individual properties.

Structure of liquid ethylene glycol: A molecular dynamics simulation study with different force fields

The structure of liquid ethylene glycol at room temperature is examined by performing molecular dynamics (MD) simulation studies for several different liquid phase force fields. We compare the properties obtained and analyze the differences which arise from the use of these models. A thorough study of molecular conformation and intermolecular structure for the different potential models is carried out given that three of the studied force fields have the same intermolecular parameters and different intramolecular interactions. In addition, the effect of molecular shape on the intermolecular structure is discussed. Due to the important role played by the highly directional forces occurring in hydrogen bonded systems, in their intermolecular structure and in the macroscopic properties of the system, we pay special attention to the analysis of the features of the hydrogen bonding patterns present in the liquid. Revealing an overall agreement with the available structural experimental data, the results obtained show that, for the simulated models, the intermolecular structure is rather similar. The dynamics of the system is studied through the self-diffusion coefficients and, in contrast to the structural properties, the results obtained for the distinct models are quite different.

Dynamics in hydrogen bonded liquids: water and alcohols

Molecular dynamics (MD) has been revealed as a powerful tool to investigate the structure and dynamics of hydrogen bonded liquids. This paper reviews recent works in which MD simulations have been used to study the influence of hydrogen bonding on different dynamic properties of liquid water and alcohols. The analysed properties include intermolecular vibrations, self-diffusion coefficients and reorientational correlation times. Finally, we present a MD study of the translational and reorientational dynamics of supercooled water at pressures up to 400 MPa. The influence of hydrogen bonding on the anomalous behaviour of the dynamic properties of liquid water at high pressures and low temperatures is discussed.

Dielectric properties of liquid ethanol. A computer simulation study

Static and dynamic dielectric properties of liquidethanol have been studied as a function of the wave-vector number by computer simulation.Molecular dynamics simulations at room temperature have been performed using the optimized potentials for liquid simulations (OPLS) potential model proposed by Jorgensen [J. Phys. Chem. 90, 1276 (1986)]. The time dependent correlation functions of the longitudinal and transverse components of the dipole density as well as the individual and total dipole moment autocorrelation functions have been calculated. The infrared spectra and the dielectric relaxation of the liquid have been also analyzed. Results have been compared with the available experimental data. Special attention has been dedicated to investigate the molecular origin of the different analyzed properties.

The polarizable point dipoles method with electrostatic damping: Implementation on a model system

Recently, the use of polarizable force fields in Molecular Dynamics simulations has been gaining importance, since they allow a better description of heterogeneous systems compared to simple point charges force fields. Among the various techniques developed in the last years the one based on polarizable point dipoles represents one of the most used. In this paper, we review the basic technical issues of the method, illustrating the way to implement intramolecular and intermolecular damping of the electrostatic interactions, either with and without the Ewald summation method. We also show how to reduce the computational overhead for evaluating the dipoles, introducing to the state-of-the-art methods: the extended Lagrangian method and the always stable predictor corrector method. Finally we discuss the importance of screening the electrostatic interactions at short range, defending this technique against simpler approximations usually made. We compare results of density functional theory and classical force field-based Molecular Dynamics simulations of chloride in water.

Molecular simulation of liquid water confined inside graphite channels: Thermodynamics and structural properties

We carried out molecular dynamics simulations to describe the properties of water inside a narrow graphite channel. Two stable phases were found: a low-density one made of water clusters adsorbed on the graphite sheets and a liquid one that fills the entire channel, forming several layers around a bulk-like region. We analyzed the interfacial structure, orientational order, water residence times in several regions, and hydrogen bonding of this last water phase, calculating also a quantity of electrochemical interest, the probability of electron tunneling through interfacial water. The results are in good qualitative agreement with the available experimental data.

Effects of concentration on structure, dielectric, and dynamic properties of aqueous NaCl solutions using a polarizable model

The study of NaCl solutions in water at finite concentration, explicitly including polarization in water molecules and ions, has been carried out by molecular dynamics simulations. A comparison of the RPOL polarizable model with the rigid SPC/E potential for water has been included. Structure obtained with the two models does not show significant differences, although some deviations in the NaNa radial distribution functions at all concentrations are observed. Dielectric properties such as total and molecular dipole moment correlation functions revealed decay times of the order of 10 ps, roughly independent of concentration. The analysis of electric conductivity by means of current-current correlation functions also included the calculation of cross terms corresponding to dipole moment-current correlations, which proved to be non-neglectable at short times and especially relevant at high concentrations (m=4 mol kg(-1)). Frequency dependent dielectric constants and conductivities have been computed and the role of cross correlations has been analyzed. In all cases both concentration and cross correlations have significant influence in the results.