Symmetry of a molecule in an external field

It is stated that, although an external field distorts a rigid molecule, it does not destroy internal geometric symmetry of its dynamics, which is specified by a point group. As a consequence, degeneracies of stationary states connected with this group are preserved. This statement makes it possible to describe the spectrum of a molecule in an external field based only on symmetry principles, which is exemplified by the linear Zeeman effect.