Chemistry Reference
In-Depth Information
quadrupole interactions. The Huggins and Mayer force field thus has the
following form:
q
i
q
j
r
ij
þ
C
ij
r
ij
D
ij
r
ij
ð
Þ¼
B
ij
exp
ð
a
Þ
½
1
U
ij
r
ij
ij
r
ij
where
i
and
j
can be either a cation or anion, and
U
ij
is the potential energy
of species
i
due to species
j
, which are separated from each other by a distance
r
ij
. The van der Waals terms were parameterized by Mayer.
17
Tosi and Fumi
18
developed parameters for the repulsive part of this function to reproduce the
solid-phase properties of alkali halides having the rock salt structure. Many
authors subsequently used this parameterization (or variations of it) to simulate
a wide range of alkali halides in both the solid and molten states. Sangster and
Dixon
19
have reviewed much of the early work in this field, and their study is an
excellent starting point for those interested in these systems. The Tosi-Fumi
potential still enjoys wide use today.
20,21
It is generally thought that static
properties are captured fairly well with this model, but its ability to model
dynamic properties accurately has been questioned recently due to its neglect
of ion-induced polarization. Work by Galamba and co- workers
22-24
suggests
that shear viscosities and thermal conductivities of molten NaCl and KCl are
over predicted by 10-20% with this force field. There are also questions as to
how well this model reproduces static properties away from the state point at
which it was parameterized. For example, a comparison of the pressure depen-
dence of the melting point for NaCl computed with the Tosi-Fumi force field
25
and experimental data
26
is given in Figure 4. Although the Tosi-Fumi force field
Figure 4
Computed (circles)
25
solid-liquid coexistence curve for NaCl versus
experimental data (triangles).
26
Although agreement is good at atmospheric pressure,
the slope of the coexistence curve is off and thus the melting point is overestimated
severely at high pressure.
