Tabulation and interpolation in the treatment of intermolecular interactions

By default DL_POLY_4 tabulates in memory most of the intermolecular interactions keeping values of the potential and the negative of its first derivative times the distance (or virial) over an equidistant grid. This is done for reasons of speed as due to the large variety of potential forms, some could be quite expensive to evaluate if run unoptimised. The memory tabulation could be overridden for non-tabulated interactions upon user specified options such as metal direct for metal interactions and vdw direct for van der Waals interactions.

When energy and force are calculated for tabulated interactions a 3-point interpolation scheme of our own is used to interpolate the value for the requested distance.

A 5-point interpolation is used for finding the numerical derivatives of (2B)(E)EAM type potentials which are supplied in TABEAM by the user. For this a Lagrange formula is used, which can be found in any textbook on numerical methods.