Ab initio MO and approximate density functional theory studies on the lowest singlet and triplet states of s and as‐indacene

The lowest singlet and triplet states of s ‐ and as ‐indacene have been studied by means of high‐level ab initio MO and approximate density functional methods. Among the geometrical and energetical details discussed are the equilibrium structure of s ‐indacene ( C 2 h or D 2 h ), the structures and energies of the low‐lying s ‐indacene triplet states, and the stability and geometry of the singlet and triplet states of as ‐indacene. It is shown that single‐determinant‐based methods, such as Hartree‐Fock or MP 2, are not suited to properly describe these molecules. Instead, methods are required which explicitly take into account nondynamical and dynamical electron correlation. The results obtained by density functional theory‐based methods compare very well with the most elaborate ab initio MO data and seem to provide an economical alternative even for molecules with a complicated electronic structure such as s ‐ and as ‐indacene.