The classical virial expansion expresses the pressure
P
{\displaystyle P}
of a many-particle system in equilibrium as a power series in the number density:
Z
≡
P
R
T
ρ
=
A
+
B
ρ
+
C
ρ
2
+
⋯
{\displaystyle Z\equiv {\frac {P}{RT\rho }}=A+B\rho +C\rho ^{2}+\cdots...