Gluon field strength correlation functions within a constrained instanton model

We suggest a constrained instanton (CI) solution ill the physical QCD vacuu m which is described by large-scale vacuum field fluctuations. This solutio n decays exponentially at large distances. It is stable only if the interac tion of the instanton with the background vacuum field is small and additio nal constraints are introduced. The CI solution is explicitly constructed i n the ansatz form, and the two-point vacuum correlator of the gluon field s trengths is calculated in the framework of the effective instanton vacuum m odel. At small distances the results are qualitatively similar to the singl e instanton case; in particular! the D-1 invariant structure is small, whic h is in agreement with the lattice calculations.