Introduction

N-body simulations involve a large number of decisions and the situation becomes even more complex when astrophysical processes are added. The guiding principle of efficient code design must be to provide a framework for decision-making that is sufficiently flexible to deal with a variety of special conditions at the appropriate time. Since the direct approach is based on a star-by-star treatment at frequent intervals, this prerequisite is usually satisfied. However, we need to ensure that the relevant tests are not performed unnecessarily. The development of suitable criteria for changing the integration method or identifying procedures to be carried out does in fact require a deep understanding of the interplay between many different modes of interactions. Hence building up the network for decision-making is a boot-strapping operation needing much patience and experience. The aim of a good scheme should be that this part of the calculation represents only a small proportion of the total effort.

This chapter discusses several distinct types of decisions necessary for a smooth performance. First we deal with the important task of selecting the next particle, or block of particles, to be advanced in time. The challenge is to devise an optimized strategy in order to reduce the overheads. Another aspect concerns close encounters, either between single particles or where one or more subsystems already consist of binaries.