The input parameters are the source spectrum, the gas composition, and the gas density or pressure. The spectrum of the central source of radiation is described by the spectral luminosity, , where is the total luminosity (in erg s). The spectral function, , is normalized such that and may be of one of a variety of types, including: Thermal bremsstrahlung, ; blackbody, ; or power law, ; or the user may define the form of the ionizing continuum by providing a table of energies and fluxes. The gas consists of the elements H, He, C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni with relative abundances specified by the user. The default abundances are the solar values given by [Grevesse, Noels and Sauval 1996].