  A non-perturbative calculation of the ghost-gluon running QCD coupling
constant is performed using $N_f=2$ twisted-mass dynamical fermions. The
extraction of $\Lambda_{\bar{MS}}$ in the chiral limit reveals the presence of
a non-perturbative OPE contribution that is assumed to be dominated by a
dimension-two $\VEV{A^2}$ condensate. In this contest a novel method for
calibrating the lattice spacing in lattice simulations is presented.
