The breaking of molecular point group symmetry

Abstract In this review paper the definition of the elements of the molecular point group are carefully considered. It is shown that certain terms in the molecular Hamiltonian are not invariant to these operations and, hence, these terms can break the point group symmetry. The symmetry labelling of the energy levels of H2, H2O and CH4 are studied in detail in order to demonstrate this point. It is shown that the rotational energy levels of molecules can be satisfactorily labelled using the Complete Nuclear Permutation Inversion Group or the Molecular Symmetry Group as appropriate.