  The running coupling constant can be estimated by computing gluon two- and
three-point Green functions from the lattice. Computing in lattice implies
working in a fixed gauge sector (Landau). An source of systematic uncertainty
is then the contribution of non gauge-invariant condensates, as $<A^2 >$,
generating power corrections that might still be important at very high
energies. We study the impact of this gluon condensate on the analysis of gluon
propagator and vertex lattice data and on the estimate of $\Lambda_{\bar{\rm
MS}}$. We finally try a qualitative description of this gluon condensate
through the instanton picture
