To examine how decreasing hydration affects the performance of OPLS3e, we compared the headgroup order parameters S β CH and S α CH from OPLS3e and CHARMM36 simulations against experimental NMR data (Fig. ). In experiments, S β CH and S α CH rise when hydration level drops, a change that should be captured in simulations. Although the S β CH are not within +/-0.02 from experimental data (Fig. ), which indicates that neither force field exactly produces the atomistic resolution structural ensemble of the headgroup, the changes produced in S Based on these data, structural response to dehydration seems quite realistic for the headgroup in OPLS3e. Botan et al. suggest an intuitive explanation for the rising headgroup order parameters of the headgroup to be the choline headgroup orientating more parallel to the membrane as the interlamellar space shrinks in response to a decreasing hydration level . To conclude: in response to dehydration, OPLS3e does not produce atomistic resolution but its performance is very similar to CHARMM36 and headgroup orientation in OPLS3e can be thought to be reasonably accurate under dehydrated conditions. Although, many currently available force fields produce qualitatively correct response to lowering hydration, OPLS3e and CHARMM36 seem to be among the most realistic considering the magnitude of the response. In the experiments, adding NaCl induces minimal decrease to S α CH and S β CH, suggesting minimal Na + binding to membrane . Several MD force fields overestimate Na + binding, and consequently S α CH and S β CH drop significantly more than in the experiments . Based on the headgroup order parameter change (Fig. left panels) and ion distributions (Fig. ) in our simulations, OPLS3e is not an exception: Na + binding is overestimated. Notable is that overestimation is visible also at lower concentrations, near the physiological 150 mM concentration, that has the highest relevance in life sciences. Shapes of the order parameter curves in response to rising concentration of NaCl are very similar in OPLS3e and CHARMM36 (Fig. left panels). Also, distributions of Na + and Cl -seem highly similar ( Distributions of Na + and Cl -(Fig. ) show that at lower salt concentrations (100 and 200 mM) ions are not only accumulated in the vicinity of the membrane, but they are also unevenly distributed in the bulk solution (Na + and Cl -curves do not converge in the bulk); meaning that even with the rather large water/lipid ratios used here, the desired effect of having equal concentrations of both ions in the bulk solution is not reached in either of the forcefields with [NaCl] < 500 mM. Overestimation of ion binding in OPLS3e can be seen also in ion distributions, especially for CaCl 2 (Fig. ). Membrane pulls all calcium ions from the solution at low (≤ 200 mM) concentrations (and nearly all at higher concentrations), leaving no Ca 2+ ions to the bulk. A high density of Ca 2+ ions can be seen at the membrane surface, and consequently, the neutral PC bilayer will appear as positively charged, pulling a high density of Cl -ions next to the Ca 2+ . This charge layering will result in a strong electrostatic gradient. Such an artificially charged membrane can distort MD simulation results; in addition to effects on membrane behavior, influence on charged domains of membrane proteins is also conceivable, and might underlie some contradicting MD simulation results . Therefore, extreme caution should be exercised when simulating membrane protein systems containing ions using force fields that are known to overestimate cation binding.