As an example application of using the NMRlipids Databank to evaluate conformational dynamics of a large set of simulation data, Fig. shows the effective rotational correlation times of lipid C-H bond vectors, τ e , calculated from all simulations in a preliminary version of the databank: 102 Only Slipids and CHARMM36 force fields are in line with the experimental data, predicting conformational timescales below 5 ns for all lipid segments. Other tested force fields exhibit significantly slower dynamics in the glycerol backbone and headgroup regions, but such discrepancies are not observed in the acyl chain region. This provides further support for the results from the NMRlipids Project indicating that CHARMM36 gives the most realistic description for the lipid headgroup region, while all force fields capture acyl chain region properties with reasonable accuracy. Another application of the NMRlipids Databank is exemplified in Fig. , which shows the area per lipid calculated from all POPC:POPS mixture simulations currently available in the NMRlipids Databank. The area per lipid relates to the lateral density and is commonly used to characterize membrane properties. Most force fields predict a decreased area per lipid Figure : Area per lipid calculated from all simulations of POPC:POPS mixtures at 298 K currently available in the NMRlipids Databank (left) ordered according to the quality of the POPS sn-1 acyl chain (right). The number in parenthesis describes the negative 10-base logarithm of the average probability of the acyl chain CH 2 -group order parameters to reside within experimental error in a simulation, thereby decreasing with increasing simulation quality. For reference, the values extracted from the experimental X-ray scattering data using the scattering density profile (SDP) model are shown in black. (i.e., tighter membrane packing) with an increasing amount of charged POPS lipids in the membrane, yet the slope of the decrease depends on the force field parameters (Fig. ). Since the area per lipid is not a directly measurable quantity in experiments, we quality-evaluate the results using the acyl chain order parameters. These depend on the membrane order, thereby being a good proxy for the area per lipid. As all force fields predict acyl chain order parameters of pure POPC bilayers with reasonable accuracy, we focused on evaluating the quality of POPS acyl chain order parameters. The qualities of POPS simulations are ranked on the right-hand side of Fig. based on C-H bond order parameters of the CH 2 groups in the sn-1 acyl chain of POPS. The quality of POPS simulations systematically decreases with decreasing area per lipid-from the GROMOS-CKP simulation with the largest area per lipid to the Berger simulation with the smallest area per lipid and the worst agreement with experiment. The quality evaluation based on acyl chain order parameters agrees with the comparison of area per lipid values extracted from X-ray scattering experiments using the SDP model 104,105 (Fig. ). The force field dependent counterion binding affinities can explain the differences in the simulated POPS lipid bilayer areas: The bound ions screen the electrostatic repulsion between charged PS headgroups, thus leading to decreasing area per lipid with increasing binding affinity. In conclusion, all force fields seem to overestimate to varying extent the counter ion binding affinity to POPS lipids, except for the GROMOS-CKP simulations. However, GROMOS-CKP predicts an unexpected minimum of area per lipid for a mixed PC:PS system, which cannot be verified with the currently available experimental data.