Consequently, the interaction may be characterised by a dimensionful parameter Λ, namely the value of the RG scale at which the coupling constant diverges. In the case of quantum chromodynamics, this energy scale Λ is called the QCD scale, and its value 220 MeV supplants the role of the original dimensionless coupling constant in the form of the logarithm (at one-loop) of the ratio μ and Λ. Perturbation theory, which produced this type of running formula, is only valid for a (dimensionless) coupling g ≪ 1. In the case of QCD, the energy scale Λ is an infrared cutoff, such that μ ≫ Λ implies g ≪ 1, with μ the RG scale.