C. Martín

An integral equation study of a simple point charge model of water

We present an extensive integral equation study of a simple point charge model of water for a variety of thermodynamic states ranging from the vapor phase to the undercooled liquid. The calculations are carried out in the molecular reference-hypernetted chain approximation and the results are compared with extensive molecular dynamics simulations. Use of a hard sphere fluid as a reference system to provide the input reference bridge function leads to relatively good thermodynamics. However, at low temperatures the computed microscopic structure shows deficiencies that probably stem from the lack of orientational dependence in this bridge function. This is in marked contrast with results previously obtained for systems that, although similarly composed of angular triatomic molecules, do not tend to the tetrahedral coordinations that are characteristic of water.

Phase behavior of attractive and repulsive ramp fluids : Integral equation and computer simulation studies

Using computer simulations and a thermodynamically self-consistent integral equation we investigate the phase behavior and thermodynamic anomalies of a fluid composed of spherical particles interacting via a two-scale ramp potential (a hard core plus a repulsive and an attractive ramp) and the corresponding purely repulsive model. Both simulation and integral equation results predict a liquid-liquid demixing when attractive forces are present, in addition to a gas-liquid transition. Furthermore, a fluid-solid transition emerges in the neighborhood of the liquid-liquid transition region, leading to a phase diagram with a somewhat complicated topology. This solidification at moderate densities is also present in the repulsive ramp fluid, but in this case inhibits the fluid-fluid separation.

Demixing in binary mixtures of apolar and dipolar hard spheres

We study the demixing transition of mixtures of equal size hard spheres and dipolar hard spheres using computer simulation and integral equation theories. Calculations are carried out at constant pressure, and it is found that there is a strong correlation between the total density and the composition. The critical temperature and the critical total density are found to increase with pressure. The critical mole fraction of the dipolar component on the contrary decreases as pressure is augmented. These qualitative trends are reproduced by the theoretical approaches that on the other hand overestimate by far the value of the critical temperature. Interestingly, the critical parameters for the liquid-vapor equilibrium extrapolated from the mixture results in the limit of vanishing neutral hard sphere concentration agree rather well with recent estimates based on the extrapolation of charged hard dumbbell phase equilibria when dumbbell elongation shrinks to zero [G. Ganzenmuller and P. J. Camp, J. Chem. Phys. 126, 191104 (2007)].

On the use of semiphenomenological closures in integral equations for classical fluids

We have studied two choices of semiphenomenological closures for the Ornstein–Zernike equation, both for a monoatomic Lennard‐Jones fluid and a dipolar homonuclear hard diatomic fluid. One of the closures was originally proposed by Verlet for hard‐sphere systems, for which is known to yield good results. A second closure is proposed by us in the frame of the reference hypernetted chain (RHNC) theory. We have described the reference systems in this closure by means of Verlet’s approximation and its recent extension to systems of nonspherical particles. This second approach, which we denote by RHNC‐VM (Verlet’s modified), turns out to give an excellent description of the structure and thermodynamics of the Lennard‐Jones fluid and very accurate predictions for the structure of the dipolar diatomic system. In this latter case the apparent superiority of hypernetted chain results for configurational energies is found to stem from fortuitous cancellation of errors in the integration of the components of the pai...

An accurate theoretical description of fluids composed of fully anisotropic molecules: Application to C2v symmetry

We use two molecular integral equation approximations to compute the thermodynamic properties and microscopic structure of two liquids composed of planar molecules with C2v symmetry, namely SO2 and H2S. These approximations couple the exact molecular Ornstein–Zernike equation with the hypernetted chain (HNC) and reference-hypernetted chain (RHNC) closures. The theoretical results obtained for various thermodynamic states agree remarkably well with molecular dynamics calculations. In particular, the atom-atom distribution functions are very well reproduced. We find that the RHNC approximation with a hard-sphere fluid reference system offers notable improvement over HNC in the pressure calculation. We include also a self-consistent mean field calculation to incorporate the effect of polarizability on the dielectric constant of liquid SO2. Final results for this quantity are in excellent agreement with experimental values. In contrast, the model used for the electrostatic interactions in H2S leads to anomalo...

Atomic structure factors from a molecular integral equation theory: An application to homonuclear diatomic fluids

We present a novel approach for the theoretical determination of atomic structure factors (or site–site distribution functions) based on the calculation of the molecular pair distribution function by integral equations (reference hypernetted chain approximation). The results are compared with experimental structure factors and computer simulation results for homonuclear diatomic fluids (N2, Cl2 and Br2) which are modeled by means of two center Lennard‐Jones potentials. The proposed method leads to a surprisingly good agreement with experimental data, within the obvious limitations that stem from intrinsic inadequacies of the model interaction potential. Comparison with RISM integral equation results evidences the superiority of the molecular integral equation approach.

Integral equation and simulation studies of a realistic model for liquid hydrogen chloride

Liquid hydrogen chloride is modeled by a system of heteronuclear two‐center Lennard‐Jones particles with embedded point dipoles and quadrupoles. The effect of molecular polarizability is incorporated via an effective dipole approximation. The study is performed by Monte Carlo reaction field simulation and by hypernetted chain and reference hypernetted chain integral equations. Our simulation results yield dielectric properties in excellent agreement with experimental data for liquid HCl. As for the integral equation approach, we have experimented with an empirical choice of the reference system in the spirit of a recently proposed treatment which has proved extremely successful for pure and quadrupolar two‐center Lennard‐Jones fluids. The hypernetted chain equation performs slightly better when accounting for the multipolar contributions to the configurational energy, but as a whole the reference hypernetted chain equation, as introduced, here proves to be a more appropriate choice.

Study of dipolar fluid inclusions in charged random matrices

Structural, thermodynamic, and dielectric properties of a dipolar fluid confined in a charged random matrix are studied by means of grand canonical Monte Carlo simulation and replica Ornstein–Zernike integral equations in the hypernetted chain approximation. The fluid is modeled by a system of dipolar hard spheres. Two matrix topologies are considered: a frozen restricted primitive model matrix and a frozen hard sphere fluid with randomly distributed negative and positive charges. Both models lead to similar results in most cases, with significant deviations from the behavior of the corresponding equilibrated mixtures. The dielectric behavior is particularly interesting, since the effect of partial quenching on the equilibrated mixture recovers the electrostatics of the pure dipolar fluid but with the presence of Coulomb tails in the dipole–dipole total correlations. Differences between the two matrix models arise more vividly in the low density regime, in which the matrix with randomly distributed charge...

Hypernetted and reference hypernetted chain integral equations for Lennard‐Jones diatomic fluids with quadrupolar interactions

The hypernetted chain and reference hypernetted chain integral equations are solved for quadrupolar two‐center Lennard‐Jones fluids and the computed fluid structure and thermodynamics are contrasted with computer simulation. A reference bridge function is determined through an empirical modification of Verlet’s approximation. The resulting reference hypernetted chain equation leads to good agreement with simulation data. On the contrary, the bare hypernetted chain approximation performs poorly, in particular as far as the equation of state is concerned, which is a well‐known drawback of this closure when short ranged repulsive potentials come into play.

Structure and thermodynamics of heteronuclear two-centre Lennard-Jones fluids from Monte Carlo simulation and a reference hypernetted chain equation

We have performed extensive Monte Carlo (MC) simulations to evaluate structural and thermodynamic properties of heteronuclear two-centre Lennard-Jones fluids. Computations have been carried out for two uncharged molecular models which roughly describe liquid HCl and HBr. These simulations are used as test benchmarks for a reference hypernetted chain approximation (RHNC-VM), which is the natural extension to heteronuclear fluids of a previously developed approach fairly successful in the context of homonuclear diatomic fluids. The proposed theoretical approach leads to results in good agreement with MC simulation.