ABSTRACT
A rigid molecule is defined as being such that the barriers between its versions are insuperable and, as a result, there are no observable tunnelling splittings. In section 8.2, the MS group of a rigid molecule was defined as the subgroup of the complete nuclear permutation inversion (CNPI) group obtained by deleting unfeasible operations. For a rigid molecule an unfeasible operation is one that causes a coordinate change that moves the molecule from one version to another. To generalize the definition of the MS group to non-rigid molecules, we generalize the definition of an unfeasible operation:
The MS group of a molecule is the subgroup of the CNPI group obtained by removing unfeasible operations, where an unfeasible operation causes a coordinate change that moves the molecule from one version to another across an insuperable energy barrier.
