  Scattering power (T = d/dx of mean squared multiple Coulomb scattering (MCS)
angle), as used in proton transport theory, is properly viewed as a
differential description of the Gaussian approximation to MCS theories such as
Moliere's. That is, we seek a function T which, when integrated over a finite
slab, will recover the Moliere/Fano/Hanson angle for that slab. To be accurate,
T must include a single scattering correction, which means mathematically it
must be nonlocal, depending on how much MCS has taken place as well as the
energy and scattering material at the POI. We review five formulas for T and
introduce a sixth, testing each against the Moliere/Fano/Hanson prediction as
well as experimental data. We discuss how sensitive some practical problems are
to the choice of T. That choice is probably most important for general Monte
Carlo codes, which are expected to address a wide variety of problems.
