The two-loop relation between the bare lattice coupling and the MS coupling in QCD with Wilson fermions

We calculate the two-loop relation between the coupling renormalized in the MS scheme and the bare lattice coupling g0 in QCD with Wilson fermions, extending the pure gauge results of Lüscher and Weisz. Our calculation leads to the the third coefficient of the lattice beta function. We show how this coefficient modifies the scaling function on the lattice.