  ``Glauber'' models are used to calculate geometric quantities in the initial
state of heavy ion collisions, such as impact parameter, number of
participating nucleons and initial eccentricity. The four RHIC experiments have
different methods for Glauber Model calculations, leading to similar results
for various geometric observables. In this document, we describe an
implementation of the Monte Carlo based Glauber Model calculation used by the
PHOBOS experiment. The assumptions that go in the calculation are described. A
user's guide is provided for running various calculations.
