A state-averaged, multireference complete active space (CAS) approach was used for the determination of the vertical excitation energies of valence and Rydberg states of para-benzyne. Orbitals were generated with a 10- and 32-state averaged multiconfigurational self-consistent field approach. Electron correlation was included using multireference configuration interaction with singles and doubles, including the Pople correction for size extensivity, multireference averaged quadratic coupled cluster (MR-AQCC) and MR-AQCC based on linear response theory. There is a very high density of electronic states in this diradical system—there are more than 17 states within 7 eV of the ground state including two 3s Rydberg states. All excitations, except 2 1Ag. are from the π system to the δδ*system. Of the 32 states characterized, 15 were multiconfigurational, including the ground 1Ag state, providing further evidence for the necessity of a multireference approach for p-benzyne. The vertical singlet-triplet splitting was also characterized using a two-state averaged approach. A CAS (2,2) calculation was shown to be inadequate due to interaction with the π orbitals.

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Copyright © 2008, AIP Publishing. This article first appeared in The Journal of Chemical Physics: 129:4 (2008), 129-136.

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