In Lattice QCD simulations numerical estimates of expectation value of observables are obtained at a finite value of lattice spacing. Hence, they are affected by cut-off effects which disappear in the continuum limit. In particular, Dirac-Wilson fermions introduce cut-off effects linear in the lattice spacing due to the Wilson term. In order to increase the precision of final, continuum extrapolated, results, we would like to construct observables which converge to their continuum limit with a^2. This can be achieved following Symanzik's improvement program in which higher dimensional operators have to be included in order to cancel the unwanted contributions linear in the lattice spacing. In this talk I will present a method of determining numerical coefficients accompanying those operators based on position space representation of correlation functions.
