In this paper we study the Casimir energy of QCD within the Gribov-Zwanziger
approach. In this model non-perturbative effects of gauge copies are properly
taken into account. We show that the computation of the Casimir energy for the
MIT bag model within the (refined) Gribov-Zwanziger approach not only gives the
correct sign but it also gives an estimate for the radius of the bag.Comment: 5 pages, No figures. The discussion about the constraints on the RGZ
parameters (as well as about the corresponding estimates) imposed by the
requirement to have the correct behaviour of the Casimir energy has been
expanded. Version accepted for publication on Physical Review
