Handbook of Solvents - George Wypych - ChemTech - Ventech!

456 Jacopo Tomasi, Benedetta Mennucci, Chiara Cappelli
choices make the results indirectly relevant to the question of the difference between traditional
and effective intermolecular potential energies. However, some hints can be extracted
from the available data. The experience thus far collected shows that when both M
and S have a polar nature, the classical electrostatic contributions are dominant with an exchange-repulsion
term, giving an important contribution to the energy but a small effect on
the charge distribution of M and, as a consequence, on the molecular properties on which
the interaction potential depends (e.g., multipole distribution of the charge density, static
and dynamic polarizabilities). Dispersion effects play a role when M is not polar and they
become the dominant component of the solvation effects when both M and S are not polar.
The calculation of effective PCM interaction potentials has been so far limited to the
cases in which the solvent effects are more sizeable, namely the interactions of metal cations
with water. They have been used in computer simulations of models of very dilute
ionic solutions (a single ion in water), noticeably improving the results with respect to similar
studies using traditional potentials. 34
8.6 THE VARIETY OF INTERACTION POTENTIALS
In the preceding sections we have examined the definition of the components of the
intermolecular potential, paying attention mostly to rigorous methods and to some approximations
based on these methods.
The beginning of qualitative and semi-quantitative computational studies on the properties
of liquids dates back at least to the 1950s, and in the early years of these studies, the
computational resources were not so powerful as they are today. For this reason, for many
years, the potentials used have been simple, discarding what our understanding of the
dimeric interactions suggested. There is no reason, however, to neglect here these simplified
versions of the potential, because very simple and naïve expressions have given excellent
results and are still in use. The struggle for better and more detailed potentials is
justified, of course, because chemistry always tends to have more and more accurate estimates
of the properties of interest, and there are many problems in which a semi-quantitative
assessment is not sufficient.
We shall try to give a cursory view of the variety of intermolecular potentials in use for
liquid systems, paying more attention to the simplest ones. We shall almost completely neglect
potentials used in other fields, such as the scattering of two isolated molecules, the determination
of spectroscopic properties of dimers and trimers, and the accurate study of
local chemical interactions, which all require more sophisticated potentials.
The main criterion to judge the quality of a potential is a posteriori, namely, based on
the examination of the performance of that potential in describing properties of the liquid
system. In fact, the performances of a potential are to a good extent based on subtle factors
expressing the equilibrium reached in the definition of its various components. The experience
gained in using this criterion has unfortunately shown that there is no definite answer.
Typically, a potential, or a family of potentials, better describe some properties of the liquid,
while a second one is more proper for other properties. For this reason there are many potentials
in use for the same liquid and there are in the literature continuous comparisons among
different potentials. A second criterion is based on the computational cost, an aspect of direct
interest for people planning to make numerical studies but of little interest for people
only interested in the results.