for understanding the phase transformations in hadronic matter. In general, a nontrivial global minimum corresponds to a gauge field with the strength not vanishing at space-time infinity and, hence, extensive action proportional to the four dimensional space-time volume of the system, unlike the localized configurations. A variety of essentially equivalent statements of the problem in the context of QCD can be found, for instance, in [@Minkowski; @Pagels; @Mink; @Leutwyler; @NK1; @Faddeev0]. Global minima related by discrete symmetry transformations like CP, Weyl symmetry in the root space of $su(N_{\rm c})$, center symmetry in particular, is a reason to look for field configurations interpolating between them. First of all, these are domain wall configurations, but also lower dimensional topological defects.

There is among others one difference between this treatment and approaches based on localized objects: the last one intends to merge the initially isolated objects (e.g., instanton gas or liquid) while the former collects defects in an initially homogeneous background. At first sight, both ways seem to lead to a similar outcome - a class of nonperturbative gluon field configurations with a self-consistent balance of order and disorder which can be characterized, in particular, by nonzero gluon condensate and topological charge density. 