  We explore the behavior of neon, xenon, and methane filmas adsorbed on the
external surface of a bundle of carbon nanotubes. The methods used are
classical: a ground state calculation, by grand potential energy minimization,
and the grand canonical Monte Carlo (GCMC) method of simulation. Our results
are similar to those found recently in a GCMC study of Ar and Kr. At low
chemical potential (pressure) the particles form a quasi-one dimensional phase
within the groove formed by two contiguous tubes. At higher chemical potential,
there occurs a "three-stripe" phase aligned parallel to the groove (except for
xenon). This is followed by monolayer and bilayer phases. The low temperature
monolayer phase is striped; the number of stripes per nanotube is a quantized
function of the adatom size. In the neon case, the bilayer regime also includes
a second layer groove phase. Our results are compared with recent thermal and
diffraction experiments. We find no evidence of a zig-zag phase reported
recently.
