GROMMET: An N-body code for high-resolution simulations of individual galaxies

This paper presents a fast, economical particle-multiple-mesh N-body code optimized for large-N modelling of collisionless dynamical processes, such as black hole wandering or bar-halo interactions, occurring within isolated galaxies. The code has been specially designed to conserve linear momentum. Despite this, it also has variable softening and an efficient block-time-step scheme: the force between any pair of particles is calculated using the finest mesh that encloses them both (respecting Newton's third law) and is updated only on the longest time-step of the two (which conserves momentum). For realistic galaxy models with N ≳ 10 6, it is faster than the fastest comparable tree code by factors ranging from ∼2 (using single time-steps) to ∼10 (multiple time-steps in a concentrated galaxy). © 2007 The Author. Journal compilation © 2007 RAS