Field form of perturbation theory for calculating the matrix elements of operators between the states of two degenerate atomic levels. I

It is shown that the calculation of the matrix elements between degenerate states of arbitrary atomicmolecular systems can be reduced to a calculation of the matrix elements for some auxiliary operator. The perturbation theory for this calculation can be given a field form, and its matrix elements can be expressed in terms of the contributions from special Feynman diagrams. The exact rules are formulated for constructing the contributions from these diagrams.