  We present a novel global 3-D coronal MHD model called COCONUT, polytropic in
its first stage and based on a time-implicit backward Euler scheme. Our model
boosts run-time performance in comparison with contemporary MHD-solvers based
on explicit schemes, which is particularly important when later employed in an
operational setting for space weather forecasting. It is data-driven in the
sense that we use synoptic maps as inner boundary input for our potential field
initialization as well as an inner boundary condition in the further MHD time
evolution. The coronal model is developed as part of the EUropean Heliospheric
FORecasting Information Asset (EUHFORIA) and will replace the currently
employed, more simplistic, empirical Wang-Sheeley-Arge (WSA) model. At 21.5 Rs
where the solar wind is already supersonic, it is coupled to EUHFORIA's
heliospheric model. We validate and benchmark our coronal simulation results
with the explicit-scheme Wind-Predict model and find good agreement for
idealized limit cases as well as real magnetograms, while obtaining a
computational time reduction of up to a factor 3 for simple idealized cases,
and up to 35 for realistic configurations, and we demonstrate that the time
gained increases with the spatial resolution of the input synoptic map. We also
use observations to constrain the model and show that it recovers relevant
features such as the position and shape of the streamers (by comparison with
eclipse white-light images), the coronal holes (by comparison with EUV images)
and the current sheet (by comparison with WSA model at 0.1 AU).
