  Using the effective potential approach for composite operators, we have
formulated a general method of calculation of the truly nonperturbative
Yang-Mills vacuum energy density (the Bag constant apart from the sign, by
definition). It is the main dynamical characteristic of the QCD ground state.
We define it as an integration of the truly nonperturbative effective charge
over the nonperturbative region (soft momentum region). It is free of all types
of the perturbative contributions, by construction. For the considered truly
nonperturbative effective charge it is finite, negative and it has no imaginary
part (stable vacuum), as well as it is a manifestly gauge-invariant, i.e., not
explicitly depending on the gauge-fixing parameter. A nontrivial minimization
procedure makes it possible to determine the Bag constant as a function either
of the mass gap, which is responsible for the large-scale structure of the true
QCD vacuum, or of the effective scale, which separates the nonperturbative
region from the perturbative one. We have also argued that the Bag constant is
a quant of energy density which can be released from the QCD vacuum, which, in
its turn, is considering as an infinite and permanent reservoir of energy.
