Format of zeeman_constants.xml:

@ Isotope GS Spin Hund_case

where GS is a constant for the molecule that calculates the effective g-value,
Spin is a molecular constant, and Hund_case tells the program to estimate the
calculations with a specific Hund case.

The Zeeman split is calculated with changes in energy levels. Essentially, 
dE = g M C_Z,
where 
g = GS/2 * g_H,
and g_H is a function of the quantum numbers for a certain transition and 
certain Hund case, M is the quantum number projection of the total angular 
momentum on the magnetic field, and C_Z is a function of physical constants and
the magnetic field.

The change in frequency is then calculated from 
df = dE'-dE'',
where the primes tells you that the quantum numbers are before and after the 
transition.

Hund case a is number "0", and 
Hund case b is number "1".

Assuming the electron is free, then GS = 2.0023193043622.
We used this assumption when we had no other data for GS. However, we do not
know how well this assumption holds for the respective molecules. Currently,
this regards NO2 and HO2.

The GS values for O2, NO, OH, ClO, and SO are from 

H. Christensen, and L. Veseth, On the High-Precision Zeeman Effect in 02 and SO.
Journal of Molecular Spectroscopy 72, 438-444, 1978.

L. Veseth, Relativistic Corrections to the Zeeman Effect in Diatomic Molecules.
Journal of Molecular Spectroscopy 66, 259-271, 1977.