Handbook of Solvents - George Wypych - ChemTech - Ventech!

460 Jacopo Tomasi, Benedetta Mennucci, Chiara Cappelli
Table 8.6. Many-site simple potentials
# Shape Interaction Name
1 three fused spheres hard hard dumbbell HD
2 two half spheres + cylinder hard hard spherocylinder HSC
3 general convex shape hard hard convex core HCC
The largest number of solvents are composed by polyatomic molecules of small-medium
size, exhibiting a variable (but in general not excessive) degree of flexibility. In solutions,
the interactions involve these solvents and molecules having the same characteristics
as solvent or with more complex chemical composition. In all cases (pure liquids, solutions
with solute of variable complexity), one has to take into account interactions having a remarkable
degree of specificity. The chemical approach to the problem addresses this specificity
on which the whole chemistry is based. This is the real field of application of the
definition and analysis of molecular interactions on which we have spent the first sections
of this chapter.
It is clear that we are entering here into a very complex realm, hard to summarize. A
whole book would be necessary.
It is possible to make a rough distinction between potential of general applicability and
potentials conceived for specific couples (or collections) of molecules.
In both cases, the many-site expansions are used, but for general potentials, in which
transferability is asked, the description is obviously less detailed. The LJ+charge (potential
14 in Table 8.5) is a popular choice for these potentials; the sites are in general limited to the
heavy atoms of the molecule. There are, however, many other versions with a variety of
changes to this standard setting.
The potentials elaborated for specific cases are often more detailed and they adapt a
larger number of devices to increase the accuracy. Many have been developed for pure liquids,
but the number of potentials regarding specific combinations of solute and solvent is
not negligible.
The number of solvents in chemistry is quite large. The number of available specific
potentials is by far more limited, but too large to be summarized here. In addition, a simple
list of names and references, not accompanied by critical remarks about the performances of
such potentials in describing liquid systems would have little utility.
The numerous textbooks on intermolecular potentials are of little help, because outdated
or paying little attention to the analytical exposition of computational models. More
useful are monographs and reviews regarding computer simulations of liquids 39 or original
papers making comparisons among different potentials.
These last are more abundant for water. This solvent, because of its importance and its
molecular simplicity, has been the benchmark for many ideas about the description and
modeling of intermolecular interactions. Almost all potentials that have been thus far proposed
for not extremely flexible molecular component of liquid systems may be found in
the literature regarding water. Alternative strategies in the defining number and position of
sites, use diffuse instead of point charges, different approaches to model the many-body
corrections, flexible potentials, dissociation of the molecule are some features that can be
examined by looking at the water potential literature.