A MODIFIED TREE CODE - DONT LAUGH - IT RUNS

I describe a modification of the Barnes-Hut tree algorithm together with a series of numerical tests of this method. The basic idea is to improve the performance of the code on heavily vector-oriented machines such as the Cyber 205 by exploiting the fact that nearby particles tend to have very similar interaction lists. By building an interaction list good everywhere within a cell containing a modest number of particles and reusing this interaction list for each particle in the cell in turn, the balance of computation can be shifted from recursive descent to force summation. Instead of vectorizing tree descent, this scheme simply avoids it in favor of force summation, which is quite easy to vectorize. A welcome side-effect of this modification is that the force calculation, which now treats a larger fraction of the local interactions exactly, is significantly more accurate than the unmodified method. © 1990.