  Understanding the cosmic ray (CR) ionization rate is crucial in order to
simulate the dynamics of, and interpret the chemical species observed in
molecular clouds. Calculating the CR ionization rate requires both accurate
knowledge of the spectrum of MeV to GeV protons at the edge of the cloud as
well as a model for the propagation of CRs into molecular clouds. Some models
for the propagation of CRs in molecular clouds assume the CRs to stream freely
along magnetic field lines, while in others they propagate diffusively due to
resonant scattering off of magnetic disturbances excited by MHD turbulence
present in the medium. We discuss the conditions under which CR diffusion can
operate in a molecular cloud, calculate the local CR spectrum and ionization
rate in both a free-streaming and diffusive propagation model, and highlight
the different results from the two models. We also apply these two models to
the propagation through the ISM to obtain the spectrum seen by Voyager 1, and
show that such a spectrum favors a diffusive propagation model.
