MolMod – an open access database of force fields for molecular simulations of fluids

The MolMod database is presented, which is openly accessible at http://molmod.boltzmann-zuse.de andcontains intermolecular force fields for over 150 pure fluids at present. It was developed and is maintainedby the Boltzmann-Zuse Society for Computational Molecular Engineering (BZS). The set of molecularmodels in the MolMod database provides a coherent framework for molecular simulations of fluids.The molecular models in the MolMod database consist of Lennard-Jones interaction sites, pointcharges, and point dipoles and quadrupoles, which can be equivalently represented by multiple pointcharges. The force fields can be exported as input files for the simulation programmes ms2 and ls1mardyn, GROMACS, and LAMMPS. To characterise the semantics associated with the numericaldatabase content, a force field nomenclature is introduced that can also be used in other contexts inmaterials modelling at the atomistic and mesoscopic levels. The models of the pure substances thatare included in the database were generally optimised such as to yield good representations ofexperimental data of the vapour–liquid equilibrium with a focus on the vapour pressure and thesaturated liquid density. In many cases, the models also yield good predictions of caloric, transport,and interfacial properties of the pure fluids. For all models, references to the original works in whichthey were developed are provided. The models can be used straightforwardly for predictions ofproperties of fluid mixtures using established combination rules. Input errors are a major source oferrors in simulations. The MolMod database contributes to reducing such errors

Molecular Simulation Study of Dielectric Constants of Pure Fluids and Mixtures

The static dielectric constant of fluids is studied with molecular models from the literature. The employed molecular models were developed using only vapor-liquid equilibrium data. No information on the dielectric properties was used, so that the simulation results are predictions. A wide range of different fluids, from slightly to strongly polar, is investigated. Most of the studied models underestimate the dielectric constant, which can be explained by the way the models were developed. For the pure fluids dimethyl ether and acetone, the temperature and pressure dependence of the dielectric constant are also studied. A good agreement with experimental data is found. Additionally, binary mixtures are investigated. Thereby, the validity of several mixing rules for the dielectric constant is assessed.

Power-law coarsening in network-forming phase separation governed by mechanical relaxation

A space-spanning network structure is a basic morphology in phase separation of soft and biomatter, alongside a droplet one. Despite its fundamental and industrial importance, the physical principle underlying such network-forming phase separation remains elusive. Here, we study the network coarsening during gas-liquid-type phase separation of colloidal suspensions and pure fluids, by hydrodynamic and molecular dynamics simulations, respectively. For both, the detailed analyses of the pore sizes and strain field reveal the self-similar network coarsening and the unconventional power-law growth more than a decade according to ℓ ∝ t1/2, where ℓ is the characteristic pore size and t is the elapsed time. We find that phase-separation dynamics is controlled by mechanical relaxation of the network-forming dense phase, whose limiting process is permeation flow of the solvent for colloidal suspensions and heat transport for pure fluids. This universal coarsening law would contribute to the fundamental physical understanding of network-forming phase separation.

Boiling Pure Fluids at Sub Atmospheric Pressures

The study and sizing of sorption machine evaporators are based on the prediction of the heat transfer coefficient at atmospheric pressures, but in the literature we only find correlations modeled from experiments for a wide range of pressure, where the majority of the data are above atmospheric pressure; A review of the experiments of boiling at sub-atmospheric pressures was carried out and compared to four known correlations for three types of fluids, which are water, hydrocarbons and refrigerants; The results obtained showed deviations of the predicted data from the experimental values for three correlations and convincing results for the fourth.