  We present a simple coarse-grained bead-and-spring model for lipid bilayers.
The system has been developed to reproduce the main (gel-liquid) transition of
biological membranes on intermediate length scales of a couple of nanometres
and is very efficient from a computational point of view. For the solvent
environment, two different models are proposed. The first model forces the
lipids to form bilayers by confining their heads in two parallel planes. In the
second model, the bilayer is stabilised by a surrounding gas of "phantom"
solvent beads, which do not interact with each other. This model takes only
slightly more computing time than the first one, while retaining the full
membrane flexibility. We calculate the liquid-gel phase boundaries for both
models and find that they are very similar.
