  We present here the first of a series of papers aimed at better understanding
the evolution and properties of giant molecular clouds (GMCs) in a galactic
context. We perform high resolution, three-dimensional {\sc arepo} simulations
of an interacting galaxy inspired by the well-observed M51 galaxy. Our fiducial
simulations include a non-equilibrium, time-dependent, chemical network that
follows the evolution of atomic and molecular hydrogen as well as carbon and
oxygen self-consistently. Our calculations also treat gas self-gravity and
subsequent star formation (described by sink particles), and coupled supernova
feedback. In the densest parts of the simulated interstellar medium (ISM) we
reach sub-parsec resolution, granting us the ability to resolve individual GMCs
and their formation and destruction self-consistently throughout the galaxy. In
this initial work we focus on the general properties of the ISM with a
particular focus on the cold star-forming gas. We discuss the role of the
interaction with the companion galaxy in generating cold molecular gas and
controlling stellar birth. We find that while the interaction drives
large-scale gas flows and induces spiral arms in the galaxy, it is of secondary
importance in determining gas fractions in the different ISM phases and the
overall star-formation rate. The behaviour of the gas on small GMC scales
instead is mostly controlled by the self-regulating property of the ISM driven
by coupled feedback.
