As shown (Figs. and), the lipid tail structures in OPLS4 are closer to experiments than in CHARMM36, whose overestimation of the S CH magnitude indicates overly ordered tails, and consequently underestimation of area per lipid. Indeed, in all dehydrated states, CHARMM36 area estimates are lower than OPLS4 by 1-3 Å2 (Fig. , Table ). Such overshrinking and overestimated tail S CH magnitudes in CHARMM36 have been previously reported, e.g., for DPPC (1,2dipalmitoylphosphatidylcholine) bilayers. To visualize how the dehydration-induced increase of S CH magnitude (Fig. ) and the concomitant decrease in area per lipid (Fig. ) affect the saturated tail geometry, Fig. shows histograms of tail end-to-end distance and tail tilt angle at decreasing hydration: As expected, the tails become straighter and orient more along the bilayer normal, but interestingly the distributions change little until < 10 w/l, and markedly only at 5 w/l. OPLS4 versus OPLS3e. Upon publishing OPLS4, 14 there were no reported changes from the older OPLS3e 53 force field concerning lipid parameters. We found that, at full hydration, the S CH in these two force fields are indeed identical (Fig. ), and they do not significantly differ even when high concentrations of salts are added (Figs. and). At low water levels, however, there seem to be small differences, especially within the headgroup and glycerol backbone (Fig. ). Most notice-ably, these lead to the dehydration-induced forking artefact of the α carbon segment (as in CHARMM36, see Fig. ) that did not appear in OPLS3e (Fig. ).