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.