  Using the effective potential approach for composite operators we have
formulated the quantum model of the QCD vacuum. It is based on the existence
and importance of the nonperturbative $q^{-4}$-type dynamical, topologically
nontrivial excitations of the gluon field configuration. The QCD vacuum is
found stable since the vacuum energy density has no imaginary part. Moreover, a
possible ground (stationary) state of the nonperturbative Yang-Mills (quenched
QCD) vacuum is discovered. The vacuum energy density at stationary state
depends on a scale at which nonperturbative effects become important. The quark
part of the vacuum energy density depends in addition on the constant of
integration of the corresponding Schwinger-Dyson equation. The value of the
above mentioned 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
order of magnitude bigger than the instanton based models can provide while a
fair agreement with recent phenomenological and lattice results for the chiral
condensate is obtained.
