public class BnPY_1QC_Helium
Computes the Percus-Yevick compressibility-route approximation for to the
first quantum correction of Bn, where the first quantum correction is
as defined Kim and Henderson (1968,1966). The gradient of u is equal
to d2u/dr2 + 2/r*(du/dr).
Considers the same potential as Kim and Henderson: Lennard-Jones parameterized
for helium by Haberlandt.
Percus-Yevick approximation is fully accurate at second and third order, and values
match those of Kim and Henderson to three or four sig figs at second and third orders.
Applicable only for spherically symmetric pair potentials.