Long Ranged Electrostatic (Coulombic) Potentials

DL_POLY_2 incorporates several techniques for dealing with long ranged electrostatic potentials. ^2.2 These are as follows.

1. Atomistic and charge group implementation.
2. Direct Coulomb sum;
3. Truncated and shifted Coulomb sum;
4. Coulomb sum with distance dependent dielectric;
5. Ewald sum;
6. Smoothed Particle Mesh Ewald (SPME);
7. Hautman Klein Ewald for systems with 2D periodicity;
8. Reaction field;
9. Dynamical shell model.

Some of these techniques can be combined. For example 1, 3 and 4 can be used in conjunction with 9. The Ewald sum, SPME and Hautman Klein Ewald are restricted to periodic (or pseudo-periodic) systems only, though DL_POLY_2 can handle a broad selection of periodic boundary conditions, including cubic, orthorhombic, parallelepiped, truncated octahedral, hexagonal prism and rhombic dodecahedral. The Ewald sum is the method of choice for periodic systems. The other techniques can be used with either periodic or non-periodic systems, though in the case of the direct Coulomb sum, there are likely to be problems with convergence.

DL_POLY_2 will correctly handle the electrostatics of both molecular and atomic species. However it is assumed that the system is electrically neutral. A warning message is printed if the system is found to be charged, but otherwise the simulation proceeds as normal. No correction for non-neutrality is applied.