Constant THERMAL_SAMPLING 

pub const THERMAL_SAMPLING: &str = "per-encounter, uncorrelated: each lattice site met along a \
     straight flight line gets one isotropic Gaussian displacement (standard deviation u1 per \
     Cartesian axis, Debye model) drawn from the history's thermal stream; redrawn after every \
     collision";

How Thermal displacements are sampled, as recorded in ThermalMetadata::sampling.