Recent Updates to the Charmm Lipid Force Fields

Since its release in the early 1990s, the CHARMM lipid force field (FF) has been updated frequently to agree with experimental data and to work with state-of-art simulation protocols. The most recent all-atom version, C36, has been heavily used due to the diversity of lipids it covers and the well-designed parameters, but it is not suitable for monolayer simulations due to the inconsistency of surface tension with experiment. To fix this problem, we introduced the long-range Lennard-Jones (LJ) interactions using the LJ Particle Mesh Ewald (LJ-PME) method and optimized the C36 parameter set to work with LJ-PME. The new force field, called C36/LJ-PME, achieved excellent agreement with a wide range of experimental data for a variety of lipid types, currently including phosphatidylcholines, phosphatidylethanolamines, phosphatidylglycerols, ether lipids and lipids with (poly)unsaturated tails. Besides the additive, CHARMM also has a polarizable lipid force field, DRUDE. While the polarizable model can provide a more realistic representation of the lipid, parameters for the DRUDE lipid force field were not optimized to the optimum, leading to unsatisfactory membrane compressibilities. Here we also report our most recent work on the DRUDE lipid force field toward a better modeling of lipid.