Non-perturbative O(a) improvement of lattice QCD
  The coefficients multiplying the counterterms required for O($a$) improvement of the action and the isovector axial current in lattice QCD are computed non-perturbatively, in the quenched approximation and for bare gauge couplings $g_0$ in the range $0 \leq g_0 \leq 1$. A finite-size method based on the Schr\"odinger functional is employed, which enables us to perform all calculations at zero or nearly zero quark mass. As a by-product the critical hopping parameter $\kappa_c$ is obtained at all couplings considered. 
Martin Luescher, Stefan Sint, Rainer Sommer, Peter Weisz and Ulli   Wolff