public class VirialHePIXCEven
MSMC calculation for the "even" exchange component of helium-4 virial coefficients.
These pieces are single rings containing paths of two or more helium-4 atoms.
Direct sampling with sampling weight determined by reference diagram.
For additive components, reference value is the same diagram for an ideal gas
(or system with some measure of IG character.)
For nonadditive components, reference value is the additive component.
Adapted from VirialHePIXC by Shaul and Schultz