All calculations in the Three Phase ISM Tool are carried out by IDL routines.
The only tricky calculation in this tool is the computation of the average mass fractions of the two species in the interstellar medium (ISM). In the case of a stable isotope in the Clayton model, the mass fraction is a straightforward expression that is simply evaluated (see eq. (3a) of Clayton, Hartmann, and Leising 1993). For a radioactive species, we use Simpson's rule to calculate the mass fraction in the gas as given by eq. (4) of the same paper. In practice, we use IDL's built-in Simpson's rule procedure.
