However, one often does not need quantitative accuracy in regions where the repulsive interaction is very large; one simply needs to be sure that the analytic potential is repulsive enough to keep trajectories from unphysically entering such a region. A pairwise additive potential is repulsive in any such region, and designing the full potential be a pairwise potential plus corrections is a reasonable strategy. Then, having the full potential reduce to a pairwise additive repulsive potential in highly repulsive regions without having data there would be a convenient way to ensure a physical fit at close distances. Note that for nZ > 4, nZ(nZ -1)/2 is greater than 3nZ -6, so a pairwise additive potential does not depend on less coordinates than 3nZ -6, but rather it is a sum of lower-dimensional potentials (in this case a sum of 1dimensional potentials), and that is what we signify when we label it as LD.