When the atm_option control has the value 'fixed_Teff', MESA
sets T_{\rm surf} from the Eddington-grey T(\tau)
relation
T_{\rm surf}^{4} = \frac{3}{4} T_{\rm eff}^{4} \left( \tau_{\rm surf} + 2/3 \right).
The effective temperature T_{\rm eff} is specified by the
atm_fixed_Teff control; see :ref:`here <tau-surf>` for a
discussion of how the surface optical depth \tau_{\rm surf} is
evaluated.
MESA then sets P_{\rm surf} to equal the radiation pressure associated with T_{\rm surf}.