  In this paper we present the results of a large-scale numerical investigation
of structural properties of a model of cell membrane, simulated as a bilayer of
flexible molecules in vacuum. The study was performed by carrying out extensive
Molecular Dynamics simulations, in the (NVE) micro-canonical ensemble, of two
systems of different sizes (2x32 and 2x256 molecules), over a fairly large set
of temperatures and densities, using parallel platforms and more standard
serial computers. Depending on the dimension of the system, the dynamics was
followed for physical times that go from few hundred of picoseconds for the
largest system to 5--10 nanoseconds for the smallest one. We find that the
bilayer remains stable even in the absence of water and neglecting Coulomb
interactions in the whole range of temperatures and densities we have
investigated. The extension of the region of physical parameters that we have
explored has allowed us to study significant points in the phase diagram of the
bilayer and to expose marked structural changes as density and temperature are
varied, which are interpreted as the system passing from a crystal to a gel
phase.
