General Treatments of Electron Correlation in Polymers

More General Treatments of Electron Correlation in Polymers.—The introduction of excitonic states was just a simple example to show how one can go beyond the HF approximation to obtain correlated electron-hole pairs, whose energy level(s) may fall into the forbidden gaps in HF theory, and form the basis for interpretation of optical phenomena in semiconducting polymers. The schemes described until now for investigation of certain types of correlation effects (the DODS method for ground-state properties and the exciton-picture for excited states) are relatively simple from both the conceptual and computational points of view and they have been actually used at the ab initio level. It is evident, on the other hand, that further efforts are needed in polymer electronic structure calculations if we want to reach the level of sophistication in correlation studies on polymers which is quite general nowadays in molecular quantum mechanics. 

If the valence band is completely filled in a polymer (or in a molecular crystal) we can subdivide the ground-state correlation into a long range and a short range part. For the long range correlation the electronic polaron model33 can be used. It has been applied already to the periodic DNA model polycytosine (polyC),34 

Toyozawa, Prog. Theor. Phys. Kyoto), 1954, 12, 421 A. B. Kunz, Phys. Rev., 1972, 

37 For a review see T. C. Collins, in Electronic Structure of Polymers and Molecular Crystals , ed. J.-M. Andre and J. Ladik, Plenum Press, New York, 1975, p. 405. 

in Recent Advances in the Quantum Theory of Polymers , ed. J.-M, Andr6, J.-L. Bredas, J. Delhalle, J. Ladik, G. Leroy, and C. Moser, Springer Verlag, Berlin, 1979, p. 155. 

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