Molecular Models

# 1 Overview

The Molecular Model Database of the Boltzmann-Zuse Society for Computational Molecular Engineering, referred to as the MolMod database, contains materials relations (force ﬁelds) for around 150 low-molecular fluids for materials modelling using molecular dynamics (MD) and Monte Carlo (MC) solvers. The MolMod database has been published by Stephan et al. [Stephan, 2019]. These force fields are known to describe vapour-liquid equilibrium data (e.g., saturated densities, vapour pressures, enthalpies of vaporization) with a good accuracy. In most cases, correlations to experimental data for these quantities were used to parameterize these molecular models. Predictions of other properties, such as transport properties, were tested and found to be in good agreement with experimental data in many cases.

The present materials relations, i.e. force fields, can be combined with the following physics equations:
• Classical mechanical equations of motion; this is typically an MD solver.
• Partition function and phase-space density (e.g., Boltzmann distribution); this is typically an MC solver.

The present molecular models are atomistic in cases where each atom of the molecule is represented by at least one interaction site, e.g., for nitrogen, oxygen, halogens, and noble gases. In other cases, a united-atom approach is followed, yielding a mesoscopic materials model where (some or all) interaction sites correspond to groups of atoms; e.g., the 2CLJQ models for refrigerants are of the mesoscopic type. Also ion models are included in the MolMod database.

The force fields in the MolMod database are defined by rigid multi-centre Lennard-Jones 12-6 potentials with superimposed point charges, dipoles, and quadrupoles. They can be easily combined for describing mixtures using combining rules, e.g., the modified Lorentz-Berthelot rule. In some cases, multiple models for the same substance were developed, e.g., for methane (one model as a simple Lennard-Jones fluid and one as Lennard-Jones truncated & shifted). The database also contains a set of ion models that can be used for modelling electrolyte systems.

Note: The present nomenclature is registered as a semantic asset in the Taxonda dashboard of the European Materials Modelling Council (EMMC). It is based on the Review of Materials Modelling (RoMM) [de Baas, 2017].

# Units

The parameters of the presented molecular force fields are given in different forms and units for the various simulation programs. Table 1 summarizes the different units of the properties depending on the respective simulation programs. Note that the dipole moment and the quadrupole moment are given in units of Debye and Buckingham, respectively.

 Name Symbol ms2 ls1 LAMMPS Gromacs Internal Coordinates Length σ Å Å Å nm - Energy ε $\varepsilon/k_\text{B}$ in K $\varepsilon/k_\text{B}$ in K eV kJ/mol - Charge q e q $(k_\text{C}/k_\text{B})^{1/2}$ in C e e - Mass M g/mol u g/mol u - Distance M Å Å Å nm Å Angle $\theta$,$\phi$ ° ° ° ° ° Dipole$^{\mathrm{(a)}}$ μ D μ $(k_\text{C}/k_\text{B})^{1/2}$ in ÅC D - - Quadrupole$^{\mathrm{(b)}}$ Q B Q $(k_\text{C}/k_\text{B})^{1/2}$ in Å$^2$C - - -

Table 1: Overview of the different types of interaction potentials and the units of their parameters depending on the simulation program. The Boltzmann constant ($k_\text{B}$) und the Coulomb's constant ($k_\text{C}$) are each given in their SI units. $^{\mathrm{(a)}}$ is given in Debye ($\mathrm{D}=0.2082\times e \times Å$). $^{\mathrm{(b)}}$ is given in Buckingham ($\mathrm{B}=\mathrm{D}\timesÅ$).

# Naming

The molar masses and the names of the substances are adopted from the "NIST Chemistry WebBook". The corresponding IUPAC-name and further common trivial names of substances available in the MolMod database are also deposited. Such are not displayed on the webfrontend, but the search field in the list of substances atoms and molecules/ions can be used to search trivial- or IUPAC substance names.