Define initial value of the log (base 10) of the
model ionization parameter. If the density is
held constant, the [Tarter Tucker and Salpeter 1969] form is used:
. If the pressure is held constant, a version of
the Krolik, McKee, and Tarter (1981) form is used:
. Note that this differs from the original
form by using the full isotropic pressure (neutral atoms + ions + electrons
+ trapped line radiation) instead of just the pressure due to hydrogen
atoms and protons. This quantity is used in calculating the radius
of the innermost edge of the shell by inverting the parameter definition.