Thermodynamic Properties for Applications in Chemical Industry via Classical Force Fields - Multiscale Molecular Methods in Applied Chemistry - page 231

their own input and force field files as well as analysis programs to compute the
desired properties from the simulation output. Many simulation tools are in constant
development and have an increasing number of active users; thus their supported
features are constantly changing.
6 Case Study: Ammonia
Ammonia is one of the most important industrial chemicals. Due to its relevance
and its simple symmetric molecular structure, much work has been devoted to the
development of a force field that is capable of accurately predicting a broad range of
its thermodynamic properties. In the following, the capabilities of force fields fitted
to QM and vapor-liquid equilibrium data to predict other pure component proper-
ties over a wide range of states are addressed.
6.1 Force Fields
Several semi-empirical and empirical force fields have been developed for ammo-
nia [ 108 , 139 , 239 - 247 ]. In this work, some rigid, non-polarizable models opti-
mized with different parameterization strategies will be addressed. Jorgensen and
Ibrahim [ 239 ] used experimental geometric information, i.e., bond lengths and
bond angles, together with ab initio information, to devise a force field based on
one LJ 12-6 site and four point charges. They used the STO-3G minimal basis set
to calculate the energy of 250 different ammonia dimer configurations. An empiri-
cal scaling factor was adopted to account for the polarizability in the liquid
phase. Hinchliffe et al. [ 240 ] followed a similar parameterization strategy, but
employed a Morse potential for repulsion and dispersion. The parameters of the
Morse potential and the four point charges were fitted to the dimer energy surface
calculated with the 6-31G * basis set for seven different dimer configurations. The
geometric parameters were taken from experimental results. Impey and Klein [ 108 ]
re-parameterized the model by Hinchliffe et al. [ 240 ] and replaced the Morse
potential with one LJ 12-6 site located at the nitrogen nucleus to describe the
dispersive and repulsive interactions. They kept the point charges at the hydrogen
nucleus positions, but displaced the nitrogen partial charge towards the hydrogen
atoms. The parameters of this five-site model were optimized to the radial distribu-
tion function of liquid ammonia.
Krist´f et al. [ 246 ] proposed an empirical force field, fitted to experimental
molecular geometry and vapor-liquid equilibrium properties. This force field con-
sists of one LJ 12-6 site plus four partial charges. Recently, Zhang and Siepmann
[ 247 ] proposed a five-site ammonia force field based on the geometry of the Impey
and Klein [ 108 ] model. This force field also consists of one LJ 12-6 site and four
partial charges, three of them located at the hydrogen positions and one located at a