Handbook of Solvents - George Wypych - ChemTech - Ventech!

8.8 Practical applications of modeling 487
As a last note we underline that, while it is easy to see that nonequilibrium effects have
to be taken into account in the presence of fast changes in the electronic distribution of the
solute, or of an oscillating external field (as that exploited to measure molecular optical
properties), nonequilibrium approaches for nuclear vibrational analyses are still open.
8.8 PRACTICAL APPLICATIONS OF MODELING
In this section we will present some examples of how to determine properties of pure liquids
by means of the previously-shown computational procedures. In spite of the fact that most
literature in this field is concerned with the study of the properties of water, we will rather
focus the attention on the determination of the properties of a few organic solvents of common
use (for a review of studies on water, we address interested readers to the already
quoted paper of Floris and Tani). 40 Obviously, the following discussion cannot be considered
as a complete review of all the literature in the field, but it is rather intended to give the
reader a few suggestions on how the previously-shown computational methodologies can
be used to obtain data about the most usual properties of pure liquids.
From what we said in the previous section, it should be clear to the reader that continuum
models cannot properly be used in order to determine liquid properties: they are in fact
concerned in the treatment of a solute in a solvent, and so not of direct use for the study of
the solvent properties. For this reason, we will present in the following some examples concerning
the application of integral equations and computer simulations.
The analysis will be limited to the examination of the most usual static and dynamic
properties of liquids.
Dielectric constant
Let us begin the discussion by considering a couple of examples regarding the calculation of
the dielectric constant of pure liquids.
The critical role of the dielectric constant in determining the properties of a liquid and
in particular in moderating the intermolecular interactions cannot be overemphasized. It is
then extremely important that potential energy functions used, for example, in computer
simulations are able to reproduce the experimental value of the dielectric constant and that
such a calculation can be used to estimate the validity of any empirical potential to be used
in simulations. The problem of the calculation of the dielectric response is complex: 108
many theories and methods are available, giving sometimes different and contrasting results.
It is accepted that for the calculation of the dielectric constant (as well as other properties
dependent on intermolecular angular correlations) via computer simulations, it is compulsory
to include long-range electrostatic interactions in the treatment. Such interactions
can be taken into account by using either the Ewald summation approach or using the reaction
field approach. Without going into details, we simply recall here that in the Ewald approach,
electrostatic interactions are evaluated using an infinite lattice sum over all periodic
images, i.e., by imposing a pseudo-crystalline order on the liquid. In the reaction field approach,
however, electrostatic interaction beyond the spherical cutoff (see Section 8.7.2 for
details) are approximated by treating the part of the system outside the cutoff radius as a
polarizable continuum of a given dielectric constant εrf. Several methods exist for the calculation of the dielectric constant by means of computer
simulations. The most widely used is the calculation of the average of the square of the
total dipole moment of the system, (fluctuation method). Using reaction field bound-