  The launch of the Infrared Space Observatory and new atomic data have opened
a window to the study of high ionization gas in active galactic nuclei (AGN).
We present the results of a large number of photoionization simulations of the
``coronal line'' region in AGN, employing new atomic data from the Opacity and
Iron Projects. Our grid of line emission spans 8 orders of magnitude in gas
density and 14 orders of magnitude in ionizing flux in an effort to identify
the optimal conditions in which these lines form. We show that coronal lines
form at distances from just outside the broad line region to ~400L_{43.5}^{1/2}
pc, in gas with ionization parameter -2.0 < log U(H) < 0.75, corresponding to
gas densities of 10$^2$ to 10$^{8.5}$ cm$^{-3}$, with electron temperatures
~12,000K -- 150,000K. A large range of distances from the central source
implies significant line width variation among the coronal lines. We identify
several line ratios which could be used to measure relative abundances, and we
use these to show that the coronal line gas is likely to be dust free.
