Symplectic Integrator Mercury: Bug Report

TL;DR

This paper reports a bug in the MERCURY symplectic integrator used in astronomy simulations, caused by uninitialised variables, and proposes explicit initialization as a solution to ensure reliable simulations.

Contribution

Identifies a critical bug in MERCURY and demonstrates that explicit variable initialization prevents non-physical disappearance of bodies in simulations.

Findings

Uninitialised variable causes bodies to disappear unexpectedly.

Explicit initialization fixes the bug permanently.

Compiler differences affect bug manifestation.

Abstract

We report on a problem found in MERCURY, a hybrid symplectic integrator used for dynamical problems in Astronomy. The variable that keeps track of bodies' statuses is uninitialised, which can result in bodies disappearing from simulations in a non-physical manner. Some FORTRAN compilers implicitly initialise variables, preventing simulations from having this problem. With other compilers, simulations with a suitably large maximum number of bodies parameter value are also unaffected. Otherwise, the problem manifests at the first event after the integrator is started, whether from scratch or continuing a previously stopped simulation. Although the problem does not manifest in some conditions, explicitly initialising the variable solves the problem in a permanent and unconditional manner.