Gustavo A. Chapela

Molecular dynamics simulation of the orthobaric densities and surface tension of water

Molecular dynamics simulations have been performed to study the liquid–vapor equilibrium of water as a function of temperature. The orthobaric densities and the surface tension of water are reported for temperatures from 316 K until 573 K. The extended simple point charge (SPC/E) interaction potential for water molecules is used with full Ewald summation. The normal and tangential components of the pressure tensor were calculated and are presented at 328 K. The nature of the long‐range contribution to the surface tension has been studied in detail. At 328 K the calculated surface tension is 66.0±3.0 mN m−1 in comparison with the experimental value of 67 mN m−1. The simulated surface tensions between 316 K and 573 K are in good agreement with experiment. The orthobaric densities are in better agreement with experimental values than those obtained from the Gibbs ensemble calculation for the SPC model of water.

Molecular dynamics for discontinuous potentials

A method to perform molecular dynamics simulations for systems of particles interacting with discontinuous potentials is presented. It is a generalization of Alder and Wainwrights algorithm for potentials represented by an arbitrary number of discontinuous horizontal line segments. The method is applied to several hard molecular fluids of various shapes, discrete Lennard-Jonesium, surfaces and mixtures in order to test its generality and flexibility. Results are compared, when possible, with previously published material, which includes numerical, theoretical and experimental results. These comparisons show that the method is a powerful tool in the study of molecular fluids and looks like a promising alternative approach to perform molecular dynamics for continuous potentials represented by a series of discontinuous line segments. The flexibility of the method will allow the testing of theories based on integral equations, like RISM and RAM, and to develop equations of state for complex hard molecules to...

The surface tension of TIP4P/2005 water model using the Ewald sums for the dispersion interactions

The liquid-vapor phase equilibria and surface tension of the TIP4P/2005 water model is obtained by using the Ewald summation method to determine the long range Lennard-Jones and electrostatic interactions. The method is implemented in a straightforward manner into standard simulation programs. The computational cost of using Ewald sums in dispersion interactions of water is estimated in direct simulation of interfaces. The results of this work at 300 K show a dramatic change in surface tension with an oscillatory behavior for surface areas smaller than 5x5sigma(2), where sigma is the Lennard-Jones oxygen diameter. The amplitude of such oscillations substantially decreases with temperature. Finite size effects are less important on coexisting densities. Phase equilibria and interfacial properties can be determined using a small number of water molecules; their fluctuations are around the same size of simulation error at all temperatures, even in systems where the interfaces are separated a few molecular diameters only. The difference in surface tension of this work compared to the results of other authors is not significant (on the contrary, there is a good agreement). What should be stressed is the different and more consistent approach to obtain the surface tension using the Ewald sums for dispersion interactions. There are two relevant aspects at the interface: An adsorption of water molecules is observed at small surface areas and its thickness systematically increases with system size.

The short range anion-H interaction is the driving force for crystal formation of ions in water.

The crystal formation of NaCl in water is studied by extensive molecular dynamics simulations. Ionic solutions at room temperature and various concentrations are studied using the SPC/E and TIP4P/2005 water models and seven force fields of NaCl. Most force fields of pure NaCl fail to reproduce the experimental density of the crystal, and in solution some favor dissociation at saturated conditions, while others favor crystal formation at low concentration. A new force field of NaCl is proposed, which reproduces the experimental phase diagram in the solid, liquid, and vapor regions. This force field overestimates the solubility of NaCl in water at saturation conditions when used with standard Lorentz-Berthelot combining rules for the ion-water pair potentials. It is shown that precipitation of ions is driven by the short range interaction between Cl-H pairs, a term which is generally missing in the simulation of ionic solutions. The effects of intramolecular flexibility of water on the solubility of NaCl ions are analyzed and is found to be small compared to rigid models. A flexible water model, extending the rigid SPC/E, is proposed, which incorporates Lennard-Jones interactions centered on the hydrogen atoms. This force field gives liquid-vapor coexisting densities and surface tensions in better agreement with experimental data than the rigid SPC/E model. The Cl-H, Na-O, and Cl-O pair distribution functions of the rigid and flexible models agree well with experiment. The predicted concentration dependence of the electric conductivity is in fair agreement with available experimental data.

Fluid phase equilibria using molecular dynamics: the surface tension of chlorine and hexane

In this paper we demonstrate that the direct molecular dynamics method can be used to predict accurate fluid phase equilibria for molecular fluids. The method is applied to chlorine and n-hexane to calculate the coexisting densities, vapour pressure, and surface tension as a function of temperature. Chlorine is modelled as a rigid diatomic molecule, and n-hexane as an isotropic united-atom model. For hexane we use two sets of parameters for the intermolecular potential. The main difference in the parameters is the strength of the repulsion-dispersion interaction of the terminal methyl group eCH3 /k = 90·44 K (model I) and = 114 K (model II); systematic differences in the calculated properties are found for the models. For chlorine, the liquid-vapour densities and vapour pressures are in excellent agreement with experimental results, and with those previously calculated using the Gibbs ensemble Monte Carlo method (GEMC). Good agreement with the experimental surface tensions is obtained. For hexane, the cal...

Effect of flexibility on surface tension and coexisting densities of water.

Molecular dynamics simulations of pure water at the liquid-vapor interface are performed using direct simulation of interfaces in a liquid slab geometry. The effect of intramolecular flexibility on coexisting densities and surface tension is analyzed. The dipole moment profile across the liquid-vapor interface shows different values for the liquid and vapor phases. The flexible model is a polarizable model. This effect is minor for liquid densities and is large for surface tension. The liquid densities increase from 2% at 300 K to 9% at 550 K when the force field is changed from a fully rigid simple point charge extended (SPCE) model to that of a fully flexible model with the same intermolecular interaction parameters. The increases in surface tension at both temperatures are around 11% and 36%, respectively. The calculated properties of the flexible models are closer to the experimental data than those of the rigid SPCE. The effect of the maximum number of reciprocal vectors (h(z) (max)) and the surface area on the calculated properties at 300 K is also analyzed. The coexiting densities are not sensitive to those variables. The surface tension fluctuates with h(z) (max) with an amplitude larger than 10 mN m(-1). The effect of using small interfacial areas is slightly larger than the error in the simulations.

The Wolf method applied to the liquid-vapor interface of water

The Wolf method for the calculation of electrostatic interactions is applied in a liquid phase and at the liquid-vapor interface of water and its results are compared with those from the Ewald sums method. Molecular dynamics simulations are performed to calculate the radial distribution functions at room temperature. The interface simulations are used to obtain the coexisting densities and surface tension along the coexistence curve. The water model is a flexible version of the extended simple point charge model. The Wolf method gives good structural results, fair coexistence densities, and poor surface tensions as compared with those obtained using the Ewald sums method.

On the calculation of the sphericity factor for fused hard sphere molecules

A general algorithm is presented to obtain the exposed surface area, volume, mean radius of curvature and sphericity factor of a molecule composed of fused hard spheres. The procedure, based on a Monte Carlo method, is easily programed. Results are compared with published data and values for interesting molecules are given.

Discrete perturbation theory for the hard-core attractive and repulsive Yukawa potentials

In this work we apply the discrete perturbation theory [A. L. Benavides and A. Gil-Villegas, Mol. Phys. 97, 1225 (1999)] to obtain an equation of state for the case of two continuous potentials: the hard-core attractive Yukawa potential and the hard-core repulsive Yukawa potential. The main advantage of the presented equation of state is that it is an explicit analytical expression in the parameters that characterize the intermolecular interactions. With a suitable choice of their inverse screening length parameter one can model the behavior of different systems. This feature allows us to make a systematic study of the effect of the variation in the parameters on the thermodynamic properties of this system. We analyze single phase properties at different conditions of density and temperature, and vapor-liquid phase diagrams for several values of the reduced inverse screening length parameter within the interval kappa( *)=0.1-5.0. The theoretical predictions are compared with available and new Monte Carlo simulation data. Good agreement is found for most of the cases and better predictions are found for the long-range ones. The Yukawa potential is an example of a family of hard-core plus a tail (attractive or repulsive) function that asymptotically goes to zero as the separations between particles increase. We would expect that similar results could be found for other potentials with these characteristics.

Discrete perturbation theory applied to Lennard-Jones and Yukawa potentials

Discrete perturbation theory (DPT) is a powerful tool to study systems interacting with potentials that are continuous but can be approximated by a piecewise continuous function composed of horizontal segments. The main goal of this work is to analyze the effect of several variables to improve the representation of continuous potentials in order to take advantage of DPT. The main DPT parameters chosen for the purpose are the starting location and size of the horizontal segments used to divide the full range of the potential and its maximum reach. We also studied the effect of having each segment aligned to the left, to the right, or centered on the continuous function. The properties selected to asses the success of this strategy are the orthobaric densities and their corresponding critical points. Critical parameters and orthobaric densities were evaluated by DPT for each of an ample set of variables and compared with their values calculated via discontinuous molecular dynamics. The best sets of DPT parameters are chosen so as to give equations of state that represent accurately the Lennard-Jones and Yukawa fluids.