Internal conversion funnel in benzene and pyrazine: adiabatic and diabatic representation

The two lowest singlet surfaces of benzene and pyrazine exhibit a low-lying conical intersection on the reaction path to a biradical form called prefulvene. The potential-energy surfaces have been calculated with the complete-active-space self-consistent-field method. The conical intersection has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximal interstate coupling. Quasi-diabatic potential-energy surfaces have been obtained via a block-diagonalization of the CI matrix. The ab initio diabatic potentials reveal that the conical intersection is locally well described by a simple linear model in both benzene and pyrazine.