Handbook of Solvents - George Wypych - ChemTech - Ventech!

8.6 The variety of interaction potentials 457
Our exposition will not try to classify potentials according to such criteria, but simply
show the variety of potentials in use. We shall pay attention to the description adopted for a
single molecule, that must then be combined with that adopted for the interaction partners.
In particular, we shall consider the number of sites and the shape of the molecule used in
each description.
The number of sites reflects the possibility we have examined, and advocated, of using
many-center expansions to improve the representation. Each expansion center will be a site.
There are models with one, two, and more sites. This sequence of increasing complexity
reaches the number of heavy atoms of the molecule and then the whole number of atoms, including
hydrogens. It is not limited to the nuclei as expansion sites. There are potentials introducing
other locations of sites, in substitution or in addition to the nuclei. For example
potentials widely used in simulations adopt for water a four-site model; other potentials
(rarely used in simulations) prefer to use the middle of the bonds instead of (or in addition
to) nuclei. Each site of the molecule must be combined with the sites of the second (and
other) molecule to give the potential.
The shape of the molecule reflects the effect of the exchange-repulsion interaction.
For almost all many-site models the shape is not given, but it implicitly results to be that of
the union of the spheres centered on the expansion sites provided by a source of exchange-repulsion
potential. There are some simple models in which the shape is explicitly
stated. There will be spheres, ellipsoids, cylinders and more complex shapes, as fused
spheres, spherocylinders, etc. Some typical examples are reported in Table 8.5.
Table 8.5. Single site-based potentials
# Shape Interaction Name
1 Sphere hard HS
2 Sphere soft SS
3 Sphere hard with charge CHS
4 Sphere soft with charge CSS
5 Sphere hard with rigid dipole DHS
6 Sphere soft with dipole DSS
7 Disc hard HD
8 Disc hard with dipole SD
9 Ellipsoid hard
10 Ellipsoid Gay-Berne 35
11 Quadrupolar shape Zewdie 36
12 Sphere repulsion dispersion Lennard Jones (LJ)
13 Sphere repulsion dispersion Buckingham
14 Sphere LJ + charge LJ+q
15 Sphere LJ + dipole Stockmayer
16 Sphere LJ + dipole+ soft sticky SSD 37