Define model pressure in dynes cm. Note that this quantity represents the full isotropic pressure (neutral atoms + ions + electrons + trapped line radiation) instead of just the pressure due to hydrogen atoms and protons. Whether or not this quantity is held fixed is determined by the value of the constant pressure switch lcpres. If the pressure is constant then the appropriate definition of ionization parameter () is adopted. In the constant density case, lcpres=0, then this quantity is ignored.