  In this paper, a posteriori error estimates of functional type for a
stationary diffusion problem with nonsymmetric coefficients are derived. The
estimate is guaranteed and does not depend on any particular numerical method.
An algorithm for the global minimization of the error estimate with respect to
the flux over some finite dimensional subspace is presented. In numerical
tests, global minimization is done over the subspace generated by
Raviart-Thomas elements. The improvement of the error bound due to the
p-refinement of these spaces is investigated.
