M. Lombardero

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.

Integral equation algorithm for fluids of fully anisotropic molecules

We outline a practical algorithm for the solution of liquid‐state integral equations for fluids of fully anisotropic rigid molecules requiring three Euler angles for their configurational description and leading to pair functions of five angular variables. The method is suitable for all potentials. We illustrate the technique with sample results for SO2.

Perturbation theory for polyatomic fluids

A Barker-Henderson like perturbation theory for polyatomic fluids is developed. The molecular interaction forces are assumed to be described by an interaction site model potential and the reference system is a fluid of hard interaction site model molecules. The theory is used to study the equation of state of nitrogen, the theoretical results being compared with experimental data and with those coming from other theories. The agreement between theory and experiment is as good as that shown by Barker and Henderson theory for monoatomic systems.

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.

Perturbation theory for multicomponent molecular fluids

A Barker-Henderson like perturbation theory formulated for polyatomic fluids is employed to determine excess thermodynamic functions of argon + nitrogen and argon + oxygen mixtures. Molecular interactions are described by an ISM potential and the properties of the reference system are calculated from a modified version of Boubliks equation of state for hard convex bodies; the site-site correlation functions are evaluated from the RISM equation.

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.

The dipolar hard sphere fluid hypernetted chain approximation revisited

The full nonlinearized hypernetted chain (HNC) approximation is solved for a system of dipolar hard spheres. Results for structural, thermodynamic and dielectric properties are presented. Comparison with simulation and reference hypernetted chain (RHNC) approximation results shows that the HNC equation matches or even improves on the more sophisticated RHNC so far as configurational energies and dielectric properties are concerned. Nonetheless, as expected, pressures and structural properties are affected by the poor performance of the HNC approximation for highly repulsive potentials.