The lateral diffusion coeficient of lipids is closely related to the structure and phase of membranes, which is one of the most important dynamic parameters of biomembranes. From our atomistic MD simulations, we calculated the lateral diffusion coeficients of the lipid molecules in our Figure : The acyl chain order parameter of POPC in the pure POPC membrane systems (A) and mixed POPC/POPS membrane systems (B), and the acyl chain order parameter of POPS in the mixed membrane systems (C). The solid lines were obtained using the multisite Ca 2+ model (Cam) and the dashed lines were obtained using the default Ca 2+ model (Cal). model membranes from the mean square displacement of lipid phosphorus atoms (Fig. ). The diffusion constant was calculated by itting a straight line from 100 ns to 150 ns according to Eq. ( ).