CouplingConstant is an option for several Feynman rule
functions and for CovariantD and
FieldStrength. In the convention of the add-on PHI,
CouplingConstant is also the head of coupling constants.
CouplingConstant takes three extra optional arguments, with
heads RenormalizationState,
RenormalizationScheme and ExpansionState
respectively. E.g. CouplingConstant[QED[1]] is the unit
charge, CouplingConstant[ChPT2[4],1] is the first of the
coupling constants of the Lagrangian ChPT2[4].
CouplingConstant[a_,b_,c___][i_] := CouplingConstant[a,b,RenormalizationState[i],c].
Overview, CovariantD, FieldStrength.