  With the aim of establishing a criterion for identifying when a lipid bilayer
has reached steady state using the molecular dynamics simulation technique,
lipid bilayers of different composition in their liquid crystalline phase were
simulated in aqueous solution in presence of CaCl$_2$ as electrolyte, at
different concentration levels. In this regard, we used two different lipid
bilayer systems: one composed by 288 DPPC (DiPalmitoylPhosphatidylCholine) and
another constituted by 288 DPPS (DiPalmitoylPhosphatidylSerine). In this sense,
for both type of lipid bilayers, we have studied the temporal evolution of some
lipids properties, such as the surface area per lipid, the deuterium order
parameter, the lipid hydration and the lipid-calcium coordination. From their
analysis, it became evident how each property has a different time to achieve
equilibrium. The following order was found, from faster property to slower
property: coordination of ions $\approx$ deuterium order parameter $>$ area per
lipid $\approx$ hydration. Consequently, when the hydration of lipids or the
mean area per lipid are stable, we can ensure that the lipid membrane has
reached the steady state.
