  Lattice data seems to show that power corrections should be convoked to
describe appropriately the transition of the QCD coupling constant running from
U.V. to I.R. domains. Those power corrections for the Landau-gauge MOM coupling
constant in a pure Yang-Mills theory (N_f=0) are analysed in terms of Operator
Product Expansion (O.P.E.) of two- and three-point Green functions, the gluon
condensate <A^2> emerging from this study. The semi-classical picture given by
instantons can be also used to look for into the nature of the power
corrections and gluon condensate.
