Cold moderators in LEAPR about njoy21

[chapmancw]

While not directly related to solid methane, I have come across a similar problem.

I’m trying to process the JEFF3.3 thermal scattering library for para-hydrogen, and I noticed that there was some odd behavior in the energy-differential cross section. For example, with Ei=1.16 eV, the energy-differential cross section exhibits an odd behavior:

Looking at the plot, from Ef=0-0.69 eV, the curve appears to be close to flat, indicating that it is in the short collision time regime. The curious thing is that there are defined S(alpha,beta) data points in that energy range that are ignored. Looking at the THERMR source file, there’s a flag sabflg defined on line 1545 of thermr.f which is used to say that, if S(alpha,beta) is less than this value (currently set at -225), to automatically assume short-collision-time calculations (instead of calculating the cross section from tabulated S(alpha,beta)). This might be a problem, since many data points in the JEFF3.3 evaluation of para-hydrogen are less than this value (the data is stored as natural log of S(alpha,beta)).

Is there a physical reason for there to have a cutoff at -225 (10^-225 in this case), or is it an artifact of an older version of NJOY?