  Understanding the (de)mixing behavior of multicomponent lipid bilayers is an
important step towards unraveling the nature of spatial composition
heterogeneities in cellular membranes and their role in biological function. We
use coarse-grained molecular dynamics simulations to study the composition
phase diagram of a quaternary mixture of phospholipids and cholesterol. This
mixture is known to exhibit both uniform and coexisting phases. We compare and
combine different statistical measures of membrane structure to identify the
onset of phase coexistence in composition space. An important element in our
approach is the dependence of composition heterogeneities on the size of the
system. While homogeneous phases can be structured and display long correlation
lengths, the hallmark behavior of phase coexistence is the scaling of the
apparent correlation length with system size. Because the latter cannot be
easily varied in simulations, our method instead uses information obtained from
observation windows of different sizes to accurately distinguish phase
coexistence from structured homogeneous phases. This approach is built on very
general physical principles, and will be beneficial to future studies of the
phase behavior of multicomponent lipid bilayers.
