Molecular dynamics (MD) simulations of dipalmitoylphosphatidylcholine (DPPC) with 72 and 288 lipids are used to examine system size dependence on dynamical properties. Similar to pure bulk liquid simulations, the lateral diffusion constant (D_s) of the lipid depends on the system size. Contrary to bulk liquid simulations, the lipid motion is enhanced with smaller system sizes, i.e., D_s is 2.92x10^-7 and 0.95x10^-7 cm²/s for 72 and 288 lipids, respectively. This finite size effect for D_s is the result of particle-mesh Ewald (PME) introducing an artificial periodicity and correlated lipid motion for simulations with 72 lipids. For the 288 lipids system, correlated motion that propagates across the periodic boundary is absent. Moreover, a MD simulation with simple electrostatic cutoffs does not exhibit correlated lipid motion, which implies PME is the source of this enhanced lipid diffusion.

Contrary to the lateral diffusion, the internal dynamics of lipids are independent of the system size. Specifically, reorientational correlation functions for lipid rotation and the phosphate-nitrogen dipole are equivalent for the 72 and 288 lipid simulations. The calculated choline, glycerol, and aliphatic carbon ¹³C NMR relaxation times (T₁) are statistically equivalent for both simulation system sizes. Differences between simulation and experiment for the some T₁ indicate areas for further improvement of the CHARMM27r force field.