Calculations in external fields in QCD: the operator method

We develop a convenient formalism for determining the coefficients of gluonic operators in the expansion of colorless correlation functions. The method proposed generalizes the technique of Schwinger for calculating in external electromagnetic fields. All principal elements and techniques are illustrated in specific examples. For this purpose we consider in detail the problem of the determinant of the Dirac operator for heavy quarks. The answer for the quark loop is obtained in the form of an expansion in G/sub munu//sup a//m2, where G/sub munu//sup a/ is the gluon field strength tensor and m is the quark mass. All terms up to G4 are taken into account