The 2017 Release of Cloudy
  We describe the 2017 release of the spectral synthesis code Cloudy. A major
development since the previous release has been exporting the atomic data into
external data files. This greatly simplifies updates and maintenance of the
data. Many large datasets have been incorporated with the result that we can
now predict well over an order of magnitude more emission lines when all
databases are fully used. The use of such large datasets is not realistic for
most calculations due to the time and memory needs, and we describe the limited
subset of data we use by default. Despite the fact that we now predict
significantly more lines than the previous Cloudy release, this version is
faster because of optimization of memory access patterns and other tuning. The
size and use of the databases can easily be adjusted in the command-line
interface. We give examples of the accuracy limits using small models, and the
performance requirements of large complete models. We summarize several
advances in the H- and He-like iso-electronic sequences. We use our complete
collisional-radiative models of the ionization of these one and two-electron
ions to establish the highest density for which the coronal or interstellar
medium (ISM) approximation works, and the lowest density where Saha or local
thermodynamic equilibrium can be assumed. The coronal approximation fails at
surprisingly low densities for collisional ionization equilibrium but is valid
to higher densities for photoionized gas clouds. Many other improvements to the
physics have been made and are described. These include the treatment of
isotropic continuum sources such as the cosmic microwave background (CMB) in
the reported output, and the ability to follow the evolution of cooling
non-equilibrium clouds.
