We use a simple and efficient computer model to investigate the physicalproperties of bilayer membranes. The amphiphilic molecules are modeled as shortrigid trimers with finite range pair interactions between them. The pairpotentials have been designed to mimic the hydrophobic interactions, and toallow the simulation of the membranes without the embedding solvent as if themembrane is in vacuum. We find that upon decreasing the area density of themolecules the membrane undergoes a solid-fluid phase transition, where in thefluid phase the molecules can diffuse within the membrane plane. The surfacetension and the bending modulus of the fluid membranes are extracted from theanalysis of the spectrum of thermal undulations. At low area densities weobserve the formation of pores in the membrane through which molecules candiffuse from one layer to the other. The appearance of the pores is explainedusing a simple model relating it to the area dependence of the free energy.
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