A computer simulation for a simple model of liquid hydrogen chloride

Random packing dynamics of [Formula: see text]-triplets

In this paper, we used a combination of DEM and the multi-sphere method to investigate the random packing dynamics of [Formula: see text]-triplets. These triplets consist of three overlapping primary spheres, forming a bent structure with a bond angle of [Formula: see text] and belonging to the [Formula: see text] symmetry group. The motivation for choosing such a structural arrangement is twofold: first, to understand how bent-shaped structures influence packing dynamics, and secondly, to investigate how mesoscopic or macroscopic particles possessing symmetry similar to that found in more elementary particles impact packing observables. To ensure non-overlapping particles at the beginning of the simulations, the confinement box was divided into basic cells. Each triplet was then inserted into a basic cell with a random orientation. After that, the system is allowed to settle under gravity toward the bottom of the box. An implicit leapfrog algorithm with quaternion acceleration was used to numerically integrate the rotational motion equations. Through a molecular approach, we consider the impact of long-range cohesive forces by employing a Lennard-Jones (LJ)-type potential. Packing processes are studied assuming different long-range interaction strengths. Different quantities are studied including packing density and orientation pair correlation function. In addition, the force probability distributions in the random packing structures have been analyzed.

Anharmonic effects in the quantum cluster equilibrium method

The well-established quantum cluster equilibrium (QCE) model provides a statistical thermodynamic framework to apply high-level ab initio calculations of finite cluster structures to macroscopic liquid phases using the partition function. So far, the harmonic approximation has been applied throughout the calculations. In this article, we apply an important correction in the evaluation of the one-particle partition function and account for anharmonicity. Therefore, we implemented an analytical approximation to the Morse partition function and the derivatives of its logarithm with respect to temperature, which are required for the evaluation of thermodynamic quantities. This anharmonic QCE approach has been applied to liquid hydrogen chloride and cluster distributions, and the molar volume, the volumetric thermal expansion coefficient, and the isobaric heat capacity have been calculated. An improved description for all properties is observed if anharmonic effects are considered.

Liquid-vapor equilibrium isotopic fractionation of water: how well can classical water models predict it?

The liquid-vapor equilibrium isotopic fractionation of water is determined by molecular-based simulation, via Gibbs ensemble Monte Carlo and isothermal-isochoric molecular dynamics involving two radically different but realistic models, the extended simple point charge, and the Gaussian charge polarizable models. The predicted temperature dependence of the liquid-vapor equilibrium isotopic fractionation factors for H(2) (18)O/H(2) (16)O, H(2) (17)O/H(2) (16)O, and (2)H(1)H(16)O/(1)H(2) (16)O are compared against the most accurate experimental datasets to assess the ability of these intermolecular potential models to describe quantum effects according to the Kirkwood-Wigner free energy perturbation variant Planck's over h(2)-expansion. Predictions of the vapor pressure isotopic effect for the H(2) (18)O/H(2) (16)O and H(2) (17)O/H(2) (16)O pairs are also presented in comparison with experimental data and two recently proposed thermodynamic modeling approaches. Finally, the simulation results are used to discuss some approximations behind the microscopic interpretation of isotopic fractionation based on the underlying rototranslational coupling.

Ab initio molecular dynamics of liquid hydrogen chloride

We carried out an ab initio molecular dynamics simulation of liquid hydrogen chloride (l-HCl) at a temperature of 313 K. Comparison with inelastic neutron scattering data shows that the simulation achieves an overall good description of the structural correlations, improving significantly upon a description based on classical interaction potentials. Despite some minor differences between theory and experiment in the H-H partial structure factor, the simulation gives a description of the hydrogen bonding in impressive agreement with experiment, for both the amount and the bond-length distribution of the bonds. In the simulation, 40% of the molecules are nonbonded, while the hydrogen-bonded chains are short, principally consisting of dimers (25%) and trimers (15%). Neighboring molecules in the simulation are found to form L-shaped arrangements, like in the isolated (HCl)(2) dimer and in crystalline phases of HCl. The time correlation of the molecular-axis orientation is found to be characterized by a very short decay time (0.13 ps), consistent with the short length of the hydrogen-bonded chains. Other dynamical properties investigated in this work include the diffusion coefficient and the vibrational density of states. We evaluated the molecular dipole of the HCl molecule in the liquid using a definition based on the coupling of rotational modes to an external electric field. The average dipole moment (1.53 D) derived in this way is found to be considerably larger than for the isolated molecule (1.11 D). Our results show that the dipole moment in [script-l]-HCl undergoes large fluctuations, both in orientation and in modulus. Upon the onset of an external field, such dipole fluctuations concur to reduce the fluctuations of the dielectric response.

Computer Simulation study of polar liquids: static and dynamic properties

The Berne-Pechukas-Kushick potential for linear molecules, together with a central electric point dipole, has been used as the model for polar liquids in a series of molecular dynamics computer simulations. Different simulations used Ewald summation, spherical cut off and spherical cut off with reaction field correction to sum the dipole-dipole interactions, and the relative merits of these methods are discussed. Results for thermodynamic properties and the distribution function h ∆ ( r ) are presented. Results for the autocorrelation functions for dipole orientation and angular velocity are also presented, and discussed in terms of memory functions, particularly with regard to the Steele perturbation theory. The complex, frequency-dependent dielectric constant has been calculated from the memory function of the autocorrelation function of the periodic cube dipole moment, and results are presented in the form of Cole-Cole plots. The ‘dielectric friction’ approach to polar liquids is examined against the evidence of the molecular dynamics data. Attention is drawn to the behaviour of the cross correlations between dipole orientations and their importance to the problem of dielectric relaxation is emphasized.