Different orientations give different energies (sometimes)

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Posted by Carmen Sousa on May 09, 2000 at 18:27:09:

I am studying a system of C4v real symmetry, lets say an square of 4 H atoms. There are two possible orientations in the cartesian axes, one with the atoms in between the axes (square orientation) and another with the atoms on the axes (rhombus orientation). I work in C2v symmetry and in both cases irreps B2 and B1 are degenerated. When I calculate the 1A2 state, which corresponds to a (a1)2 (b1)1 (b2)1 configuration, the energies are completely different in the two orientations. This does not happen for the 3A2 constructed from the same configuration. The 1A2 energy obtained in a C1 symmetry calculation coincides with the one obtained in the rhombus orientation.
Why does the energy of this singlet state change with the orientation?


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