We find OPLS4 to reproduce the structure of fully-hydrated PC-lipid bilayers well, even slightly better than CHARMM36 (Fig. ). This slight superiority further carries over to dehydration responses (Figs. and). On the whole, the two force fields appear extremely similar-also in their problems. For example, at full hydration they both struggle with the backbone g 1 and unsaturated tail segment S CH magnitudes. Further, their PC-headgroup structural response mechanisms to dehydration seem to be similar, but qualitatively different from experiments (Fig. ). For example, the POPC headgroup α carbon response to extreme dehydration has become practically identical in OPLS4 to that in CHARMM36 (Fig. ), although it was more realistic in the older OPLS3e (Fig. ).