Structure and Valence Spectroscopy

The GMS wave function for the ozone molecule supports the GVB(pp) picture for the ozone molecule in a straightforward way [54]. The two classical resonance structures were considered together with the biradical one in a three structure GMS-CI calculation. The ground state of the O3 molecule 

This is in perfect harmony with the theory presented in the first section. In fact, a simple qualitative application would allow an a priori deduction of the chemical structure of the O3 molecule, since the biradical is the only nondegenerate symmetric structure possible. 

The (n - 71 )1 3 excited states of the pyrazine molecule are a well-known case of wave function symmetry breaking [27,55]. An accidental degeneracy arises when one considers the valence electronic excitations within the equivalent nitrogen lone pairs. The pairs are in opposite and equivalent positions and, when there are two singly occupied orbitals in different symmetries, we will have a pair of accidentally degenerate states as shown below  

The total symmetry of the molecule is D2h, so we cannot have point group degenerate states. The accidentally degenerate states represented above have C2v symmetry. The direct product C2v S Cs recovers properly the full symmetry of the system. Depending on the particular excitation and final spin state four states can be generated. In Table 4 we compare the results of Hartree-Fock (HF), configuration interaction with singles and doubles excitations (CI-SD) and the two structure GMS calculations, with the available experimental data. 

Transition energies (eV) for the (n - 7i )1,3 excited states of the pyrazine molecule 

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