  We demonstrate that so called "infrared divergences" which have been
discussed in some publications during several years, do not appear within the
correct treatment of analytical properties of the transition amplitudes, in
particular, of the second order pole structure of the amplitudes describing the
$n - \bar n$ transition in nuclei. Explicit calculation with the help of the
Feynman diagram technique shows that the neutron-antineutron oscillations are
strongly suppressed in the deuteron, as well as in heavier nuclei, in
comparison with the oscillations in vacuum. General advantages and some
difficulties of the field theoretical methods applied in nuclear theory are
reminded for the particular example of the parity violating $np\to d\gamma$
capture amplitude.
