Cation binding to zwitterionic membrane Strong binding of Na + and Ca 2+ ions to zwitterionic PC lipid headgroups observed in most force fields do not agree with NMR or other experimental data. Ion binding in force fields with ECC is more realistic. NBFIX parameters between specific atoms have been introduced in CHARMM force field. Lipid-headgroupcholesterol interactions Direct interactions between cholesterol and lipid headgroups are observed in simulations with the Berger/Höltje force field combination, while CHARMM36 predicts only weak preference for the nearest lipid headgroups to locate over cholesterol. NMR data agrees better with CHARMM36, suggesting that headgroups are rotating more freely in the cholesterol-containing membranes, probably due to a less dense headgroup region. Calcium binding position in PS lipid headgroup CHARMM36 with NBFIX parameters suggests that Ca 2+ ions interact only with the carboxylate group of PS lipids. Without NBFIX the same force field indicates a significant binding affinity also to the phosphate region. Berger lipid force field suggests substantial binding also to the carbonyls in the acyl chains. In Amber-based force field with ECC, which has the most realistic ion binding behaviour when compared with NMR experiments, Ca 2+ binds approximately twice as likely to the carboxylate than to the phosphate moiety of PS, and binding only to acyl chain carbonyls is almost negligible.