  Usually, Monte Carlo models are validated against experimental data. However,
models of multiple Coulomb scattering (MCS) in the Gaussian approximation are
exceptional in that we have theories which are probably more accurate than the
experiments which have, so far, been done to test them. In problems directly
sensitive to the distribution of angles leaving the target, the relevant theory
is the Moliere/Fano/Hanson variant of Moliere theory. For transverse spreading
of the beam in the target itself, the theory of Preston and Koehler holds.
  Therefore, in this paper we compare Geant4 simulations, using the Urban and
Wentzel models of MCS, with theory rather than experiment, revealing trends
which would otherwise be obscured by experimental scatter. For medium-energy
(radiotherapy) protons, and low-Z (water-like) target materials, Wentzel
appears to be better than Urban in simulating the distribution of outgoing
angles. For beam spreading in the target itself, the two models are essentially
equal.
