Introduction

We now make an abrupt transition to a presentation of various algorithms utilized by the direct summation codes. Before proceeding further, it will be useful to include some practical aspects in order to have a proper setting for the subsequent more technical procedures. First we introduce the main codes that have been developed for studying different gravitational N-body problems. Where possible, the same data structure has been employed, except that the most recent versions are formulated in terms of the Hermite integration scheme. Since the largest codes are quite complicated, we attempt to describe the overall organization by tables and a flowchart to provide some enlightenment. Later sections give further details concerning input parameters, variables and data structure; each of these elements play an important role for understanding the general construction. We also discuss a variety of optional features which provide enhanced flexibility for examining different processes.

N-body codes

Before describing the characteristics of the codes, we introduce some short-hand notation to illustrate the different solution methods employed [cf. Makino & Aarseth, 1992]. Thus by ITS we denote the basic individual time-step scheme, whereas ACS defines the Ahmad–Cohen [1973] neighbour scheme. Likewise, HITS and HACS are used for the corresponding Hermite integration methods. Finally, MREG refers to the implementations of unperturbed three-body [Aarseth & Zare, 1974] and four-body chain regularization [Mikkola & Aarseth, 1990], as well as perturbed chain regularization [Mikkola & Aarseth, 1993].