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We present a series of adaptive mesh refinement (AMR) hydrodynamicsimulations of flat rotation curve galactic gas disks with a detailed treatmentof the interstellar medium (ISM) physics of the atomic to molecular phasetransition under the influence of diffuse FUV radiation fields and cosmic raybackgrounds. We explore the effects of different FUV intensities, including amodel with a radial gradient designed to mimic the Milky Way. The effects ofcosmic rays, including radial gradients in their heating and ionization rates,are also explored. The final simulations in this series achieve $4\:$pcresolution across the $\sim20\:$kpc global disk diameter, with heating andcooling followed down to temperatures of $\sim10\:$K. The disks are evolved for$300\:$Myr, which is enough time for the ISM to achieve a quasi-statisticalequilibrium. In particular, the mass fraction of molecular gas stabilizes by$\sim$200 Myr. Additional global ISM properties are analysed. Giant molecularclouds (GMCs) are also identified and the statistical properties of theirpopulations examined. GMCs are tracked as the disks evolve. GMC collisions,which may be a means of triggering star cluster formation, are counted and therates compared with analytic models. Relatively frequent GMC collision ratesare seen in these simulations and their implications for understanding GMCproperties, including the driving of internal turbulence, are discussed.

The Interstellar Medium and Star Formation of Galactic Disks. I. ISM and GMC properties with Diffuse FUV and Cosmic Ray Backgrounds