Beta-Function in QCD and Gluon Condensate
  Under assumption of singular behavior of alpha_s(q^2) at q^2 sim 0 and of large q^2 behavior, corresponding to the perturbation theory up to four loops, a procedure is considered of matching the beta-function at a boundary of perturbative and non-perturbative regions. The contribution of the non-perturbative region to the gluon condensate is calculated with varying normalization condition alpha_s(m_tau^2) = 0.29, 0.30,..., 0.36 for two different ways of definition of the non-perturbative invariant charge in the infrared region. We obtain quite consistent results for values of the gluon condensate, nonperturbative region scale q_0, and the string tension sigma. 
A.I. Alekseev, B.A. Arbuzov