Modifications for the Ewald Sum

For systems with periodic boundary conditions DL_POLY_2 employs the Ewald Sum to calculate the Coulombic interactions (see section 2.4.5).

Calculation of the real space component in DL_POLY_2 employs the algorithm for the calculation of the nonbonded interactions outlined above. The reciprocal space component is calculated using the schemes described in [44], in which the calculation can be parallelised by distribution of either $\mbox{$\underline{k}$}$ vectors or atomic sites. Distribution over atomic sites requires the use of a global summation of the $q_{i}\exp(-i\mbox{$\underline{k}$}\cdot\mbox{$\underline{r}$}_{j})$ terms, but is more efficient in memory usage. Both strategies are computationally straightforward. Subroutine EWALD1 distributes over atomic sites and is often the more efficient of the two approaches. Subroutine EWALD1A distributes over the $\mbox{$\underline{k}$}$ vectors and may be more efficient on machines with large communication latencies.

Other routines required to calculate the ewald sum include EWALD2, EWALD3 and EWALD4. The first of these calculates the real space contribution, the second the self interaction corrections, and the third is required for the multiple timestep option.