Development of a Grid-based Electrostatics Model in a Memory- Distributed Manner for DL_POLY_4

Electrostatic interactions in molecular simulations are usually evaluated by employing the Ewald summation method, which splits the summation into a short-ranged part, treated in real space and a long-range part treated in reciprocal space. For performance purposes in molecular dynamics software the latter is usually handled by SPME or P3M grid based methods both relying on 3D fast Fourier transform (FFT) as their central operation. However, the Ewald summation method is derived for model systems that are subject to 3D periodic boundary conditions (PBC) while there are many models of scientific as well as commercial interest, where geometry implies a 1D or 2D structures. Thus for systems, such as membranes, interfaces, linear protein complexes, thin layers, nanotubes, etc.; employing Ewald summation based techniques is either very disadvantageous computationally or impossible at all. Another approach to evaluate the electrostatics interactions is to solve the Poisson equation of the model-system charge distribution on a 3D special grid. The formulation of the method allows an elegant way to switch on and off the dependence on periodic boundary conditions in a simple manner. Furthermore, 3D FFT kernels are known to scale poorly at large scale due to excessive memory and communication overheads, which makes the Poisson solvers a viable alternative for DL_POLY on the road to exascale. This paper describes the work undertaken to integrate a Poisson solver library, developed in PRACE-2IP [1], within the DL_POLY_4 domain decomposition framework. The library relies on a unique combination of bi-conjugated gradient (BiCG) and conjugated gradient (CG) methods to warrant both independence on initial conditions with a rapid convergence of the solution on the one hand and stabilization of possible fluctuations of the iterative solution on the other. The implementation involves the development of procedures for generating charge density and electrostatic potential grids in real space over all domains in a distributed manner as well as halo exchange routines and functions to calculate the gradient of the potential in order to recover electrostatic forces on point charges.