Z = PV/NkT = 1 + B₂(T)/V + B₃/V² + ... — real gas behavior
The virial expansion expresses compressibility factor Z = PV/NkT as a density series.
B₂(T) = b − a/kT is the second virial coefficient; at the Boyle temperature T_B, B₂=0 and the gas behaves most ideally.
Van der Waals is a truncated virial with B₂=b−a/kT, B₃=b².