
Perturbation Theories for Molecular Fluids 235
where
N is the total number of monomers, either bonded or unbonded
F
0
is the free energy of the reference fluid
F
bond
is the bonding contribution to the free energy
The simplest case is a system of HS with one bonding site A, so that only dimers can
be formed. For this system, Wertheim (1984b) showed that at equilibrium
F
bond
Nk
B
T
= ln X −
1
2
X +
1
2
, (8.77)
where X = ρ
0
/ρ, the ratio of the number density of unbonded monomers ρ
0
to the
total number density ρ of monomers either bonded or unbonded. The two densities
are related by the equation
ρ = ρ
0
+4πρ
2
0
g
0
(
r
)
f
A
(
r
)
r
2
dr, (8.78)
in which f
A
(r) is the Mayer function ...