Additionally, as examples, the methane (CH 4) and H 3 molecules have highly symmetric equilibrium structures with T d and D 3h point group symmetries respectively; they lack permanent electric dipole moments but they do have very weak pure rotation spectra because of rotational centrifugal distortion. The permutation-inversion groups required for the complete study of CH 4 and H 3 are T d (M) and D 3h (M), respectively.