These are my notes on equation of state derived initially by Vardya (1965), described by Mihalas (1967) and popularized by Wittmann (1974). It is widely used (or at least present as an option) in spectral synthesis codes like SIR or NICOLE. It is also prepared for the MANCHA code with several modifications and additionally computed quantities. However, the equation is derived in this particular form to be solvable on computing resources half a century ago. From today's perspective, this formulation is rather obsolete. While the results of the VMW EOS are still largely reliable, various tricks introduced to control numerical stability limit its usability to rather restricted range of the pressure and temperature.
Here I derive a simple equation of state for the solar atmosphere following the classical work of Vardya (1965), Mihalas (1967) and Wittmann (1974). The EOS is based on the Saha ionisation equilibrium and the instanteneous chemical equilibrium for the molecules. The main ingredient is hydrogen. It's included as atomic hydrogen (H), negative hydrogen ion (H-), positive hydrogen ion (H+), and as H2 and H2+ molecules. For all other atoms the neutral and the first two ionisation stages are included.
The equations were first published by Vardya (1967). Mihalas (1967) gave a simple numerical algorithm for an efficient solution of the system. Wittmann (1974) copied the equations and add a corrective factor that provides numerical stability at high temperature.
The derivation here follows Mihalas. However, in the original derivation there is a couple of inconsistencies that obscure the procedure. Here I write the equations in a correct and consistent way.
Definitions
Let's first define the pressures:
$p_{\mathrm{H}}$ - partial pressure of the neutral H atoms;
$p_{\mathrm{H^+}}$ - partial pressure of the positive H ions (protons);
