The (β and α) headgroup S CH increase upon dehydration (Fig. ) is thought to result from the headgroup aligning more parallel to the membrane plane, and is qualitatively reproduced by most MD force fields. It seems, however, that the mechanism of this structural response differs between simulations and experiments: In experiments β changes less than α, S β CH ∼ S α CH / 3, but in simulations these roles are reversed, S β CH ∼ 3 S α CH (Fig. ). Previously such overestimation of the S β CH -versus-S α CH slope by simulations has been reported for cation binding. Possible differences in response mechanism are suggested also by the fact that only in simulations the S CH tend to fork at the lowest hydrations (particularly visible in the α carbon of POPC) and that experiments show signs of change in headgroup S CH already at > 20 w/l, but simulations only at < 20 w/l.