  We present the new general-relativistic magnetohydrodynamics (GRMHD)
capabilities of the Einstein Toolkit, an open-source community-driven numerical
relativity and computational relativistic astrophysics code. The GRMHD
extension of the Toolkit builds upon previous releases and implements the
evolution of relativistic magnetised fluids in the ideal MHD limit in fully
dynamical spacetimes using the same shock-capturing techniques previously
applied to hydrodynamical evolution. In order to maintain the divergence-free
character of the magnetic field, the code implements both hyperbolic divergence
cleaning and constrained transport schemes. We present test results for a
number of MHD tests in Minkowski and curved spacetimes. Minkowski tests include
aligned and oblique planar shocks, cylindrical explosions, magnetic rotors,
Alfv\'en waves and advected loops, as well as a set of tests designed to study
the response of the divergence cleaning scheme to numerically generated
monopoles. We study the code's performance in curved spacetimes with spherical
accretion onto a black hole on a fixed background spacetime and in fully
dynamical spacetimes by evolutions of a magnetised polytropic neutron star and
of the collapse of a magnetised stellar core. Our results agree well with exact
solutions where these are available and we demonstrate convergence. All code
and input files used to generate the results are available on
http://einsteintoolkit.org. This makes our work fully reproducible and provides
new users with an introduction to applications of the code.
