  We present a simple and highly adaptable method for simulating coarse-grained
lipid membranes without explicit solvent. Lipids are represented by one
head-bead and two tail-beads, with the interaction between tails being of key
importance in stabilizing the fluid phase. Two such tail-tail potentials were
tested, with the important feature in both cases being a variable range of
attraction. We examined phase diagrams of this range versus temperature for
both functional forms of the tail-tail attraction and found that a certain
threshold attractive width was required to stabilize the fluid phase. Within
the fluid phase region we find that material properties such as area per lipid,
orientational order, diffusion constant, inter-leaflet flip-flop rate and
bilayer stiffness all depend strongly and monotonically on the attractive
width. For three particular values of the potential width we investigate the
transition between gel and fluid phases via heating or cooling and find that
this transition is discontinuous with considerable hysteresis. We also
investigated the stretching of a bilayer to eventually form a pore and found
excellent agreement with a recently published analytic theory.
