  Context. The magnetic field in the solar atmosphere continually reconnects
and accelerates charged particles to high energies. Simulations of the
atmosphere in three dimensions that include the effects of accelerated
particles can aid our understanding of the interplay between energetic particle
beams and the environment where they emerge and propagate. We presented the
first attempt at such a simulation in a previous paper, emphasising the
physical model of particle beams. However, the numerical implementation of this
model is not straightforward due to the diverse conditions in the atmosphere
and the way we must distribute computation between multiple CPU cores. Aims.
Here, we describe and verify our numerical implementation of energy transport
by electron beams in a 3D magnetohydrodynamics code parallelised by domain
decomposition. Methods. We trace beam trajectories using a Runge-Kutta scheme
with adaptive step length control and integrate deposited beam energy along the
trajectories with a hybrid analytical and numerical approach. To parallelise
this, we coordinate beam transport across subdomains owned by separate
processes using a buffering system designed to optimise data flow. Results.
Using an ad hoc magnetic field with analytical field lines as a test scenario,
we show that our parallel implementation of adaptive tracing efficiently
follows a challenging trajectory with high precision. By timing executions of
electron beam transport with different numbers of processes, we found that the
processes communicate with minimal overhead but that the parallel scalability
is still sublinear due to workload imbalance caused by the uneven spatial
distribution of beams.
