Theory of Retinal and Its Related Compounds by the Extended INDO-CI Method Taking Account of the Change in Bond Lengths in Consistence with Electronic States

We consider retinal and its related compounds in planar configuration provided they have no ionic structure, and we extend the ordinary INDO-CI method to calculate their bond lengths in consistence with their electronic states. One of the important features of our extended INDO-CI method is that each bond length is calculated as a function of bond order which is similar to the Coulson type of relation derived by the π-electron approximation. The extended INDO-CI calculations of the bond lengths, electronic structure and optical absorptions of the molecules indicate that the previous results, which were obtained by Suzuki et al. using the π-electron approximation, are reasonable.