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Under assumption of singular behavior of invariant charge alpha_s(q^2) at q^2sim 0 and of large q^2 behavior, corresponding to the perturbation theory up tofour loops, a procedure is considered of smooth matching the beta-function at aboundary of perturbative and nonperturbative regions. The procedure results ina model for alpha_s for all q^2>0 with dimensionless parameters being fixed anddimensional parameters being expressed in terms of only one quantityLambda_QCD. The gluon condensate which is defined by the nonperturbative partof the invariant charge is calculated for two variants of ``true perturbative''invariant charge, corresponding to freezing option and to analytic one innonperturbative region. Dimensional parameters are fixed by varyingnormalization condition alpha_s(m^2_tau) = 0.29, 0.30, ..., 0.36. It isobtained that on the boundary of perturbative region alpha_0 = alpha_s(q_0^2)simeq 0.44, the procedure results in nonperturbative Coulomb componentalpha_Coulomb simeq 0.25, the nonperturbative region scale q_0 simeq 1 GeV, themodel parameter sigma simeq (0.42 GeV)^2 which suits as string tensionparameter, the gluon condensate appears to be close for two variantsconsidered, K simeq (0.33 -- 0.36 GeV)^4 (for alpha_s(m^2_tau) = 0.33).Comment: LaTeX, 18 pages, 2 figures, to be published in Mod. Phys. Lett.

AN INVARIANT CHARGE MODEL FOR ALL q2>0 IN QCD AND GLUON CONDENSATE

AN INVARIANT CHARGE MODEL FOR ALL q²>0 IN QCD AND GLUON CONDENSATE