Thermodynamic Properties for Applications in Chemical Industry via Classical Force Fields - Multiscale Molecular Methods in Applied Chemistry - page 204

explicitly included in engineering force fields because of their high computational
cost. Rather, their contributions are incorporated into pairwise approximations by
effective pair potentials [ 11 ].
In molecular force fields, the interaction energy between sites can be divided into
contributions from intramolecular and intermolecular interactions. The significance
of the different contributions to the force field varies depending on the required
application. E.g., for industrial engineering applications, simple models with a low
computational cost are required that are nonetheless able to predict accurately
thermodynamic properties. Numerous force fields of varying complexity are cur-
rently available. The simplest force fields include only potentials that describe the
intermolecular interactions and are frequently used for small molecules. More
complex force fields include intramolecular interactions that are necessary for the
simulation of larger molecules such as polymers.
All-atom force fields consider every atom as an individual interaction site,
while united-atom force fields gather different atoms of a functional group into
one interaction site, e.g., as is often done to model methyl or methylene groups.
To describe chain-like polymers or proteins, coarse grained force fields are also
employed, where the interaction sites usually represent a larger number of atoms.

2.1 Intermolecular Interactions

In modeling with classical force fields, the intermolecular interactions are usually
divided into Van der Waals interactions (repulsion and dispersion) and electrostatic
interactions. In this framework, the Van der Waals interactions take into account all
interactions between sites that are not related to permanent electrostatics, such as
dispersion, repulsion, and induction [ 12 ]. Hydrogen bonding is usually modeled by
electrostatic sites. For a detailed discussion of the intermolecular interactions, the
interested reader is referred to [ 13 ].

2.1.1 Van der Waals Interactions

The simplest potential to describe the Van der Waals interactions, neglecting
attractive forces, is the hard-sphere (HS):

r ij Þ¼ 1

r ij s;

u HS

ð (1)

0 r ij > s;

where is the site diameter and r ij is the site-site distance. A slightly more detailed

alternative to the HS potential is the soft-sphere (SS) potential:

s

v

r ij Þ¼ zðs=

r ij Þ r ij s;

u SS ð (2)

0 r ij > s;