Computes four approximations of B4 by FFT: HNC(C), HNC(V), PY(C), and PY(V).
HNC: hypernetted-chain integral-equation theory
PY: Percus-Yevick integral-equation theory
(V):virial (pressure) route
The HNC(V) approximation is equivalent to the "simple part" of the f-bond-only formulation for (at least) B4, B5, and B6.
Values agree with those computed through the recursive formalism (the Ornstein-Zernike formulation) in BnHNCLJ.java and BnPYLJ.java.
Diagrams are labeled using the nomenclature of Barker, Leonard, and Pompe (1966):
"Fifth Virial Coefficients", JCP 44(11); e.g., D4 is the four-membered ring.
X is added to the diagram name to denote if an x=r*df/dr bond is present. This our own nomenclature...