Q&A_QEC: electronic states representation

Q:  Why electronic excited states for diatomic molecules in #quantemolEC are represented differently than conventionally expected? For example, N2 molecule’s second and third excited states are represented by B1U, B3G instead of A 3∑u+, B 3∏g.   

A: This is related to Group Theory. The quantum chemical packages employed in QEC only use D_{2h} symmetry and the subgroups of D2h. Because this is a diatomic molecule, which belongs to the D_\inf h point group, the calculation has to be carried out in the lower symmetry D2h group instead.  That’s why the excited states are represented differently in the calculation results.

Figure shows the first excited state for N2 is represented as B1u, the second excited state by B3g  and degenerate states which correspond to the Pi_g state by B2g.

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