QCD with large number of quarks: effects of the instanton – anti-instanton pairs

We calculate the contribution of instanton – anti-instanton (IĪ) pairs to the vacuum energy of QCD-like theories with Nf light fermions, using the saddle point method. A qualitative change of behavior is found around Nf=6. We discuss the relation of this phenomenon to the mechanism of chiral symmetry breaking and the asymptotic of the perturbative series associated with IĪ contributions.