A large fraction of recombinations occur following cascades from a very large number of levels close to the continuum. Since explicit treatment of these levels is not feasible, we treat this process as follows (this procedure, along with detailed descriptions of other aspects of the database and the multilevel scheme are described in detail in [Bautista and Kallman 2000]): For every ion we choose a set of spectroscopic levels starting with the ground level, which are responsible for the identifiable emission lines and recombination continua; there are typically 10 - 50 of these for most ions, although for a few ions we include 100 such levels. In addition we include one or more superlevels and continuum levels. The continuum levels represent bound levels of more highly ionized species (in practice at most only a few such levels are of importance). The superlevel is an artificial level used to account for recombination onto the infinite series of levels that lie above the spectroscopic levels. In H and He-like ions the superlevels also account for the recombination cascades of these high lying levels onto the spectroscopic levels, and the rates for such decays are calculated by fitting to the results of population kinetic calculations for individual ions which explicitly include 1000 levels. For these isoelectronic sequences we explicitly include excited levels with a spectator electron, which give rise to satellite lines, excitation of these levels accounts for excitation-autoionization and radiative deexcitation, and recombination accounts for the dielectronic recombination process. For other iso-electronic sequences, the superlevels are assumed to decay directly to the ion's ground level, and the rates into and out of the superlevel are calculated in order to fit to the total recombination rates for the various ions [Bautista and Kallman 2000]. This approach allows us to simultaneously account for the contributions of excitation, ionization, and recombination to the ion's level populations. In this way we solve ionization and excitation balance without the use of total recombination rates which is customary in many nebular calculations.
By using the approach described above and providing that every transition process accompanied by its detailed balance inverse process we insure that the level populations will naturally converge to LTE under proper conditions.