EXP: N-body integration using basis function expansions

TL;DR

EXP introduces a basis function expansion method for N-body simulations that adapts to complex, non-spherical mass distributions, achieving high accuracy with less computational cost.

Contribution

The paper presents a novel basis function expansion technique tailored to non-spherical objects, improving accuracy and efficiency over traditional methods.

Findings

EXP achieves accuracy comparable to direct-summation and tree methods.

It is computationally less intensive than traditional approaches.

Numerical improvements enhance orbit integration performance.

Abstract

We present the N-body simulation techniques in EXP. 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 optimising the computation of the basis function representation. We also detail numerical improvements for performing orbit integrations, including timesteps.