  We present the MAssive ClusterS and Intercluster Structures (MACSIS) project,
a suite of 390 clusters simulated with baryonic physics that yields realistic
massive galaxy clusters capable of matching a wide range of observed
properties. MACSIS extends the recent BAHAMAS simulation to higher masses,
enabling robust predictions for the redshift evolution of cluster properties
and an assessment of the effect of selecting only the hottest systems. We study
the observable-mass scaling relations and the X-ray luminosity-temperature
relation over the complete observed cluster mass range. As expected, we find
the slope of these scaling relations and the evolution of their normalization
with redshift departs significantly from the self-similar predictions. However,
for a sample of hot clusters with core-excised temperatures
$k_{\rm{B}}T\geq5\,\rm{keV}$ the normalization and slope of the observable-mass
relations and their evolution are significantly closer to self-similar. The
exception is the temperature-mass relation, for which the increased importance
of non-thermal pressure support and biased X-ray temperatures leads to a
greater departure from self-similarity in the hottest systems. As a
consequence, these also affect the slope and evolution of the normalization in
the luminosity-temperature relation. The median hot gas profiles show good
agreement with observational data at $z=0$ and $z=1$, with their evolution
again departing significantly from the self-similar prediction. However,
selecting a hot sample of clusters yields profiles that evolve significantly
closer to the self-similar prediction. In conclusion, our results show that
understanding the selection function is vital for robust calibration of cluster
properties with mass and redshift.
