Isotropic periodic sum of electrostatic interactions for polar systems.

Isotropic periodic sum (IPS) is a method to calculate long-range interactions based on homogeneity of simulation systems. Long-range interactions are represented by interactions with isotropic periodic images of a defined local region and can be reduced to short ranged IPS potentials. The original electrostatic three-dimensional (3D)-IPS potential was derived based on a nonpolar homogeneous approximation and its application is limited to nonpolar or weak polar systems. This work derived a polar electrostatic 3D-IPS potential based on polar interactions. For the convenience of application, polynomial functions with rationalized coefficients are proposed for electrostatic and Lennard-Jones 3D-IPS potentials. Model systems of various polarities and several commonly used solvent systems are simulated to evaluate the 3D-IPS potentials. It is demonstrated that for polar systems the polar electrostatic 3D-IPS potential has much improved accuracy as compared to the nonpolar 3D-IPS potential. For homogeneous systems, the polar electrostatic 3D-IPS potential with a local region radius or cutoff distance of as small as 10 A can satisfactorily reproduce energetic, structural, and dynamic properties from the particle-meshed-Ewald method. For both homogeneous and heterogeneous systems, the 3D-IPS/discrete fast Fourier transform method using either the nonpolar or the polar electrostatic 3D-IPS potentials results in very similar simulation results.