  Using effective potential approach for composite operators we have formulated
quantum model of the QCD vacuum. It is based on the existence and importance of
the nonperturbative $q^{-4}$, topologically nontrivial excitations of the gluon
field configurations there. As a result of this the QCD vacuum is found stable
and it has a stationary state. The value of the scale reponsible for the
nonperturbative dynamics is taken from the bounds for the pion decay constant
in the chiral limit. We have obtained good numerical agreement with the
phenomenological values of the topological susceptibility, the mass of $\eta'$
meson, the gluon condensate. An excellent agreement with the phenomenological
values of the above mentioned quantities is achived by simply summing up our
contributions and instanton-induced contributions due to Negele et al to the
vacuum energy density.
