Local orbitals for the study of the π→π* excitation in polyenes

Localized orbitals have recently been employed in large ab initio calculations, but their use has generally been restricted to ground‐state problems. In this work, we analyze the molecular orbitals of the excited states, optimized with a recently proposed local procedure. This method produces local orbitals of the CAS–SCF type, which permits its application to the study of excited states. In particular, we focus on the π→π* triplet excited state in polyenes, calculated using a 2/2 CAS space which includes two electrons in one π and one π* orbitals. In small polyenes, these two singly occupied active orbitals are delocalized all along the molecule. The extent of the delocalization is analyzed by studying polyenes of increasing size. Different polyenes have been studied, going from C₁₄H₁₆ to the C₇₀H₇₂ polyene. The relation of the π→π* excitation with the cation and anion systems is also discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005