The molecular dynamics simulation systems were composed of pure POPC or mixed POPC/POPS in an aqueous CaCl 2 solution. Each single-membrane system was composed of 128 lipids and approximately 4368 water molecules, with varying concentrations of CaCl 2 (Table ). The bilayers, composed of either POPC or POPC/POPS, were symmetric in these single-membrane systems (Fig. ).