(377d) Long-Range Lennard-Jones And Electrostatic Interactions In Interfaces: Application And Development Of The Isotropic Periodic Sum Method

Molecular dynamics (MD) simulations of heptane/vapor, hexadecane/vapor, water/vapor, hexadecane/water, and dipalmitoyl phosphatidylcholine (DPPC) bilayers and monolayers are analyzed to determine the accuracy of treating long-range interactions in interfaces with the isotropic periodic sum (IPS) method. Method and cutoff (r_c) dependence of surface tensions, density profiles, water dipole orientation, and the electrostatic potential profiles are used as metrics. The water/vapor, heptane/vapor, and hexadecane/vapor interfaces are accurately and efficiently calculated with 2D IPS (r_c=10 Å). It is demonstrated that 3D IPS is not practical for any of the interfacial systems studied. However, the hybrid method PME/IPS (Particle Mesh Ewald for electrostatics and 3D IPS for Lennard-Jones (LJ) interactions) provides an accurate and efficient way to include both types of long-range forces in simulations of large liquid/vacuum and all liquid/liquid interfaces, including lipid monolayers and bilayers. For liquid/vacuum interfaces, a more efficient 2D+1D IPS method is developed with no loss of accuracy. The contribution to surface tension of LJ terms arising from interactions beyond 10 Å range from 13 dyn/cm for the hexadecane/vapor interface to approximately 3 dyn/cm for hexadecane/water, and DPPC bilayers and monolayers. Surface tensions of alkane/vapor, hexadecane/water, and DPPC monolayers based on the CHARMM lipid force fields agree very well with experiment, while surface tensions of the TIP3P and TIP4P-Ew water models underestimate experiment by 16 and 11 dyn/cm, respectively. Dipole potential drops (ΔΨ)are less sensitive to long-range LJ interactions than surface tensions. However, ΔΨ for the DPPC bilayer (845±3 mV proceeding from water to lipid) and water (547±2 mV for TIP4P-Ew and 521±3 mV for TIP3P) overestimate experiment by factors of 3 and 5, respectively, and represent expected deficiencies in non-polarizable force fields.