  Coarse-grained model for saturated (DCPC, DLPC, DMPC, DPPC, DSPC) and
unsaturated (POPC, DOPC) phospholipids is introduced within the Single Chain
Mean Field theory. A single set of parameters adjusted for DMPC bilayers gives
an adequate description of equilibrium and mechanical properties of a range of
saturated lipid molecules that differ only in length of their hydrophobic tails
and unsaturated (POPC, DOPC) phospholipids which have double bonds in the
tails. A double bond is modeled with a fixed angle of 120 degrees, while the
rest of the parameters are kept the same as saturated lipids. The thickness of
the bilayer and its hydrophobic core, the compressibility and the equilibrium
area per lipid correspond to experimentally measured values for each lipid,
changing linearly with the length of the tail. The model for unsaturated
phospholipids also fetches main thermodynamical properties of the bilayers.
This model is used for an accurate estimation of the free energies of the
compressed or stretched bilayers in stacks or multilayers and gives reasonable
estimates for free energies. The proposed model may further be used for studies
of mixtures of lipids, small molecule inclusions, interactions of bilayers with
embedded proteins.
