
182 Perturbation Theories for Fluids and Solids
but F
(0)
c
in expansions (7.17) or (7.18). From the viewpoint of the latter of these two
expansions, this approximation seems more reasonable in advance if one chooses
σ
3
0
=
i
j
x
i
x
j
σ
3
ij
, (7.21)
ε
0
=
i
j
x
i
x
j
ε
ij
, (7.22)
because in this case the first-order term in Equation 7.18 vanishes. However, as we
will see, other choices for the parameters of the reference system are also used.
On the other hand, restricting ourselves for simplicity to binary fluid mixtures,
the reference fluid may be a monocomponent fluid, one-fluid model, or an ideal
mixture of two or three fluids. Several of these models are described ...