Why polyacetylene dimerizes: Results of ab initio computations

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Abstract
Results of ab initio computations for a single chain of trans-polyacetylene are reported. Starting from self-consistent-field calculations within a Gaussian-type-orbital basis set, correlations are included by using the local ansatz. The dependence of the total energy on dimerization is then analyzed in order to explain the origin of dimerization. The position of the equilibrium dimerization is dominantly determined by atomic correlation effects. Contributions to the exchange energy from nearest-neighbor Coulomb interactions play a non-negligible role concerning the depth of the energy minimum although their strength is strongly reduced in comparison to self-consistent-field results. Electron-lattice coupling alone leads only to a marginal dimerization. These findings are in contrast to assumptions often made in connection with soliton theories.