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 pi --> pi* triplet excited state in polyenes, calculated using a 2/2 CAS space which includes two electrons in one pi and one pi* 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 pi --> pi* excitation with the cation and anion systems is also discussed. (C) 2004 Wiley Periodicals, Inc.