public class PercusYevick1QC
Computes the Percus-Yevick compressibility-route approximation 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).
Percus-Yevick approximation is fully accurate at second and third order.
Applicable only for spherically symmetric pair potentials.