The Adler-Adler representation can represent multi-channel effects, for example, the two fission channels found in even-odd fissile isotopes. It can also be Doppler broadened analytically. So far, it has only been used for one material, namely, U-233 in ENDF/B-VI. The Adler-Adler approach will probabably be entirely supplanted by the Reich-Moore representation.

Hybrid R-Function

This approach combines an R-function representation to give a full multi-level representation of elastic scattering with single-level Breit-Wigner resonance shapes in the other channels. This works well when the other channels are not strongly interacting, and it has the advantage of being able to treat charged-particle channels correctly. The hybrid approach has been used once in ENDF/B-VI.

Generalized R-Matrix

The Generalized R-Matrix approach is the ultimate representation. It can handle the full range of complexities in resonance cross sections for both light and heavy isotopes, including multi-level and multi-channel effects and charged-particle channels. However, this generality makes it very complex. There are currently no evaluations in this format, and no processing code can handle it. But the pressure is mounting to activate this generalized approach in the near future.