Chiral QCD vacuum energy density in the zero modes enhancement quantum model

Using the effective potential approach for composite operators we have form ulated the quantum model of the QCD vacuum. It is based on the existence an d importance of the nonperturbative q(-4)-type dynamical, topologically non trivial excitations of the gluon field configurations (due to self-interact ion of the massless gluons only). The QCD vacuum is found to be stable sinc e the vacuum energy density has no imaginary part. The Yang-Mills (YM) part of the vacuum energy density is always negative and depends on a finite sc ale at which nonperturbative effects become important. The quark part of th e vacuum energy density depends in addition on the constant of integration of the corresponding Schwinger-Dyson equation. The value of the above-menti oned scale is determined from the bounds for the pion decay constant in the :chiral limit. Our value for the chiral QCD vacuum energy density is one or der of magnitude bigger than the instanton based models can provide while a fair agreement with recent phenomenological and lattice results for the ch iral condensate is obtained.