Classical molecular dynamics methodology was applied for the simulation of rare gas matrix deposition and for the structural analysis of the environment around porphyrin embedded in argon or xenon. The results confirm the experimental observations of several trapping sites in argon, as opposed to one dominant location in xenon. The main site in xenon has been assigned to a symmetrical hexagonal cavity, obtained after removing seven host atoms from the {111} crystallographic plane. Differences between argon and xenon matrices are due to the fact that, for the former, a larger number of host atoms have to be removed from the crystal in order to accommodate the porphyrin molecule. This results in less stable and more perturbed site structure in argon with respect to xenon. (C) 2003 American Institute of Physics.