Energy grouping and data collapsing

Much of the data stored in a ndfy_{$\bf i$} file is a function of the incident particle's energy (e.g., total cross-section $\sigma$(E)) (and possibly outgoing transportable particle energy (e.g., transfer matrix)). As is common in deterministic computation, the energies in the ndfy_{$\bf i$} files have been discretized: the multi-energy-group approximation (see [3] page 61) which is called grouping in this document. In grouping, the incident energy range is divided up into ng regions by defining ng + 1 energy boundaries. In a ndfy_{$\bf i$} file the boundaries are stored from highest to lowest energy; the data is also stored and retrieved in that order. The incident energy groups are labeled g for 1$\le$g$\le$ng. The outgoing energy groups are label h for 1$\le$h$\le$nh. Only the transfer and fission matrix data are stored as a function of both incident and outgoing energies; their grouping and collapsing are discussed in Section 4.2. All other data is stored as a function of incident energy; their grouping and collapsing are discussed in the next section.