Several boundary conditions have been proposed to stabilize the dimerized structures of finite-length Peierls–Hubbard chains. The effects of different boundary conditions on the lattice structures and the electronic energy spectra have been analysed. It was found that the natural or the fixed-end boundary conditions are more suitable to describe polymer chains with very short lengths than the periodic boundary condition. The competition of electron–electron interactions with electron–phonon interactions in finite-length chains has also been studied.

Effect of sizes and dimensions on the spin density wave (SDW) and charge density wave (CDW) in organic ferromagnets was studied. It was found that the stable ferromagnetic state obtained in a closed ring with periodic boundary conditions would become unstable in an open chain due to the effect of chain ends. There was a competition between the spin-spin (s-s) couplings and the electron-electron (e-e) interactions for the formation of SDW and CDW in the main chain. Interchain couplings would strengthen SDW. It was suggested that the onedimensional model should be improved to understand the organic magnetism well.

The effect of atomic disorder or chain length on the stability of photoinduced polarization inversion has been studied. The atomic disorder was simulated by square-random or Gaussian-random model. It was found that for the square-random distribution case, photoinduced polarization inversion remains steadily when atomic disorder is less than 1 8a-a is the lattice constant of polyacetylene, the reversion polarization disappears and gets into normal polarized state when the disorder is stronger than over 1 8a. The reason of a normal polarization resulted from the strong lattice disorder was discussed. The relationship between the reverse polarization of biexciton state and the confinement constant te as well as the variation of chain length was also studied.

The dynamic process of charge injection is investigated for an open conjugated polymer chain in contact with a metal electrode in the framework of a nonadiabatic approach. It is found that the injected charges form wave packets due to the strong electron-lattice interactions in the conjugated polymer. A wave packet may contain any quantity of charges up to two electronic units, which depends upon the injection condition. As the system is spin degenerate within the present model, a wave packet has no spin. Discrete energy levels will appear in the gap accompanying the wave packet, which results in potential asymmetry of the charge injection and ejection between the metal electrode and the polymer. The asymmetry can be used to explain the electrical hysteresis in organic devices. It is found that there exists an upper critical field or forming field, beyond which the injected charges could not form a wave packet. Under a strong driving electric field, it is also found that a wave packet will dissociate into an extended state. The dependence of the dissociating field on the charge quantity a wave packet contains is studied.

Instead of spin electrons or holes, the carriers in polymers are supposed to be spin polarons or spinless bipolarons. Based on the spin diffusion theory and Ohm's law, we studied spin-polarized injection into organic polymers. It was obtained that only spin polarons are responsible for the current spin polarization in a ferromagnetic (FM)/organic system. Effects of the conductivity matching and the interfacial resistances on the current spin polarization were discussed.