| Summary: | The density matrix renormalization group method is applied to the Pariser-Parr-Pople-Peierls model to calculate the energies and associated structures of the low-lying states of polydiacetylene. The extrinsic dimerization of polydiacetylene, arising from the electrons in py orbitals in the triple bonds, is explicitly calculated. We find the following results. (i) Electronic interactions result in a twofold increase in the ground state dimerization, and a twofold decrease in the electronic correlation length, ξ. (ii) The vertical energy of the 21Ag+ state lies circa. 1 eV above the 11Bu- state in long chains. (iii) The 13Bu+ and 21Ag+ states undergo a sizable electron-lattice relaxation, while this is modest for the 11Bu- state. As a consequence, the relaxed energy of the 21Ag+ lies circa 0.1 eV below the relaxed energy of the 11Bu- state. (iv) The reduction in ξ results in a reversal in bond dimerizations in both the 13Bu+ and 21Ag+ states (in contrast to the noninteracting Peierls model). However, the excitonic 11Bu- state shows a polaronic distortion. We compare our results to experiment. For short oligomers the comparisons are very reasonable, but they are less satisfactory for long chains. The inclusion of solvation effects and a reparametrization of the Ohno interaction may both be necessary.
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