Atomistic and Charge Group Implementation

The Ewald sum is an accurate method for summing long-ranged Coulomb potentials in periodic systems. This can be a very cpu intensive calculation and the use of more efficient, but less accurate methods, is common. Invariably this involves truncation of the potential at some finite distance $r_{\rm cut}$. If an atomistic scheme is used for the truncation criterion there is no guarantee that the interaction sphere will be neutral and spurious ``charging'' effects will almost certainly be seen in a simulation. This arises because the potential being truncated is long-ranged ($1/r$ for charge-charge interactions). However if the cutoff scheme is based on neutral groups of atoms, then at worst, at long distance the interaction will be a dipole-dipole interaction and vary as $1/r^3$. The truncation effects at the cutoff are therefore much less severe than if an atomistic scheme is used. In DL_POLY_2 the interaction is evaluated between all atoms of both groups if any site of the first group is within the cutoff distance of any site of the second group. The groups are known interchangeably as ``charge groups'' or ``neutral groups'' in the documentation - which serves as a reminder that the advantages of using such a scheme are lost if the groups carry an overall charge. There is no formal requirement in DL_POLY_2 that the groups actually be electrically neutral.

The charge group scheme is more cpu intensive than a simple atomistic cutoff scheme as more computation is required to determine whether or not to include a set of interactions. However the size of the Verlet neighbourhood list (easily the largest array in DL_POLY_2 ) is considerably smaller with a charge group scheme than an atomistic scheme as only a list of interacting groups need be stored as opposed to a list of interacting atoms.