Energy from the nonperturbative QCD vacuum
  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. 
V. Gogokhia