QCD Perturbation Theory at Low Energies
  We apply the optimization procedure based on the Principle of Minimal Sensitivity to the third-order calculation of $\R$. The effective couplant remains finite, freezing to a value $\alpha_s/\pi = 0.26$ at low energies. Using Poggio-Quinn-Weinberg smearing we find good agreement between theory and experiment right down to zero energy. 
A.C. Mattingly and P.M. Stevenson