Effect of dehydration in OPLS4 and OPLS3e for POPC membrane. At low water level (5 waters/lipid), there seems to be variation between OPLS4 and OPLS3e in C-H bond order parameters S CH within head and glycerol group, but within tail segments force fields seem to be similar (Figs. ). The α carbon exhibits additional forking in OPLS4 (as in CHARMM36, see Fig. ), which is not appearing in OPLS3e or at higher hydration 11 states. Also, order parameters of g 3 , g 2 and g 1R carbons are diverging between OPLS4 and OPLS3e. Effect of salt in OPLS4 and OPLS3e for POPC membrane. We performed simulations with additional NaCl and CaCl 2 at 1000 mM concentration (simulation lengths 500 ns and 1 000 ns, respectively) to see whether there were changes in force field performance related to additional salt. 13