Full hydration. Figure shows the ability of OPLS4 to reproduce the experimental C-H order parameters S CH in fully hydrated DMPC (Fig. ) and POPC (Fig. ) lipid bilayers, and contrasts this with the performance of CHARMM36, a state-of-the-art force field for lipid simulations. We see that in the PC-headgroup and glycerol backbone region (left panels) OPLS4 is practically identical to CHARMM36: Just the β carbon segment S CH is 0.01 units closer to experiments in OPLS4. In the tail region (right panels) the two force fields are, however, different: Whereas CHARMM36 typically overestimates (as previously reported ) the S CH magnitudes, OPLS4 matches the experimental values, i.e., predicts slightly more disordered acyl chains than CHARMM36.