  We calculate the critical value of the hopping parameter, $\kappa_c$, in
Lattice QCD, up to two loops in perturbation theory. We employ the
Sheikholeslami-Wohlert (clover) improved action for fermions and the Symanzik
improved gluon action with 4- and 6-link loops.
  The quantity which we study is a typical case of a vacuum expectation value
resulting in an additive renormalization; as such, it is characterized by a
power (linear) divergence in the lattice spacing, and its calculation lies at
the limits of applicability of perturbation theory.
  Our results are polynomial in $c_{SW}$ (clover parameter) and cover a wide
range of values for the Symanzik coefficients $c_i$. The dependence on the
number of colors N and the number of fermion flavors $N_f$ is shown explicitly.
In order to compare our results to non perturbative evaluations of $\kappa_c$
coming from Monte Carlo simulations, we employ an improved perturbation theory
method for improved actions.
