  Monte Carlo simulation is one of the most important tools in the study of
diffusion processes. For constant diffusion coefficients, an appropriate
Gaussian distribution of particle's steplengths can generate exact results,
when compared with integration of the diffusion equation. It is important to
notice that the same method is completely erroneous when applied to
non-homogeneous diffusion coefficients. A simple alternative, jumping at fixed
steplegths with appropriate transition probabilities, produces correct results.
Here, a model for diffusion of calcium ions in the neuromuscular junction of
the crayfish is used as a test to compare Monte Carlo simulation with fixed and
Gaussian steplegth.
