  By using a novel interface between the modern smoothed particle hydrodynamics
code GASOLINE2 and the chemistry package KROME, we follow the hydrodynamical
and chemical evolution of an isolated galaxy. In order to assess the relevance
of different physical parameters and prescriptions, we constructed a suite of
ten simulations, in which we vary the chemical network (primordial and metal
species), how metal cooling is modelled (non-equilibrium versus equilibrium;
optically thin versus thick approximation), the initial gas metallicity (from
ten to hundred per cent solar), and how molecular hydrogen forms on dust. This
is the first work in which metal injection from supernovae, turbulent metal
diffusion, and a metal network with non-equilibrium metal cooling are
self-consistently included in a galaxy simulation. We find that properly
modelling the chemical evolution of several metal species and the corresponding
non-equilibrium metal cooling has important effects on the thermodynamics of
the gas, the chemical abundances, and the appearance of the galaxy: the gas is
typically warmer, has a larger molecular gas mass fraction, and has a smoother
disc. We also conclude that, at relatively high metallicity, the choice of
molecular-hydrogen formation rates on dust is not crucial. Moreover, we confirm
that a higher initial metallicity produces a colder gas and a larger fraction
of molecular gas, with the low-metallicity simulation best matching the
observed molecular Kennicutt-Schmidt relation. Finally, our simulations agree
quite well with observations which link star formation rate to metal emission
lines.
