Symmetry in (H₂)₂, (D₂)₂, (HD)₂, and H₂–D₂ Van der Waals complexes

The molecular symmetry group of the Van der Waals complex (H 2 ) 2 or (D 2 ) 2 is a group of order 16 called G 16 . This group is used to symmetry label the rotation–vibration states and the nuclear spin states of the complexes; the nuclear spin statistical weights are also determined. The group is used to classify the terms in the expansion of the dipole moment function and to determine the symmetry restrictions for allowed electric dipole transitions. The complex H 2 –D 2 has a molecular symmetry group of order 8, G 8 , and (HD) 2 a group of order 4, G 4 ; the energy level symmetries, statistical weights, and selection rules for these complexes are also determined.