NBODY2:: A direct N-body integration code

We give a full description of the code NBODY2 for direct integration of the gravitational N-body problem. The method of solution is based on the neighbour scheme of Ahmad and Cohen [J. Comput. Phys. 12 (1973) 389] which speeds up the force calculation already for quite modest particle numbers. Derivations of all the relevant mathematical expressions are given, together with a detailed discussion of the algorithms. The code may be used to study a wide variety of self-consistent problems based on a small softening of the interaction potential. (C) 2001 Published by Elsevier Science B. V.