Define model gas density, . This is actually the hydrogen nucleus density, so that, e.g., the total particle density in a fully-ionized plasma with solar abundances is . Units are cm. The default value is 1 cm. Whether or not this quantity is held fixed is determined by the value of the constant pressure switch lcpres. If the density is constant then the appropriate definition of ionization parameter () is adopted. The constant density case is the default.
Note: see the section for the radexp parameter for additional density input options.