Knowledge Bank

Previous theoretical treatments of the excited $\Pi$-electron states of benzene have included semi-empirical calculations and minimal basis set ab initio calculations. Theoretical work on ethylene and butadiene has shown the importance of including diffuse basis functions in ab initio treatments of excited state $\Pi$-electron systems. Extensive configuration interaction calculations on $\Pi$-electron states of $C6H6$ are presented here. These are based on a $\sigma$-core potential obtained in an all-electron SCF calculation on the ground state using a double zeta basis of contracted gaussians. In the CI calculations, two diffuse $\Pi$-basis functions were added on each carbon atom in order to improve the description of the excited states. Similar calculations on butadiene had predicted the existence of low-lying doubly excited states derived from the simultaneous triplet excitations of the two ethylenic units. The existence of similar states for benzene is also investigated.