ACCOUNTING FOR ELECTRON-ELECTRON AND ELECTRON-LATTICE EFFECTS IN CONJUGATED CHAINS AND RINGS

Minimum total energy calculations are reported for pi-conjugated hydro carbons including neutral (ground, 1 B-1(u), 2 (1)A(g)) and doped (1and 2+) chains and rings with up to eight carbon atoms. Two models are considered; first, a semiempirical pi-electron Hamiltonian that inclu des both electron-electron (Hubbard) and electron-lattice (Longuet-Hig gins-Salem) interactions, and second, an accurate ab initio complete-a ctive-space self-consistent-held (CASSCF) treatment that includes the pi-electron correlation effects most important in determining the bond geometries. The results of the ab initio calculations can be used to estimate the phenomenological parameters entering the semiempirical Ha miltonian and thus to obtain quantitative predictions of bond geometri es from the semiempirical treatment. The two models yield qualitativel y the same results for the bond geometries in all states considered, a nd the changes in bond geometry following excitation from ground to do ped or excited states find natural interpretation in terms of short-ch ain limiting behaviors of soliton and polaron distortions familiar for longer chains. Further, the absolute values and sensitivities of the phenomenological parameters of the semiempirical model to various fitt ing schemes provide an indication of the different roles played by ele ctron-lattice and electron-electron interactions in determining the pr operties of these systems. While electron-lattice interactions are fou nd to be the most important factor in determining bond geometries, par ticularly in the ground and doped states, electron-electron interactio ns play an important and subtle role in determining the bond geometrie s and relative energetic orderings of the excited states. (C) 1996 Ame rican Institute of Physics.