exp: N-body integration using basis function expansions

We present the N-body simulation techniques implemented in the exp code. exp uses empirically chosen basis functions to expand the potential field of an ensemble of particles. Unlike other basis function expansions, the derived basis functions are adapted to an input mass distribution, enabling accurate expansion of highly non-spherical objects, such as Galactic discs. We measure the force accuracy in three models, one based on a spherical or aspherical halo, one based on an exponential disc, and one based on a bar-based disc model. We find that exp is as accurate as a direct-summation or tree-based calculation, and in some ways is better, while being considerably less computationally intensive. We discuss optimizing the computation of the basis function representation. We also detail numerical improvements for performing orbit integrations, including time-steps.

Automated summary

This article presents the N-body simulation techniques implemented in the exp code, which uses empirically chosen basis functions to expand the potential field of a ensemble of particles. The research finds that the method can accurately expand highly non-spherical objects and achieves comparable or even better accuracy than direct-summation or tree-based calculations, while being computationally less intensive.