Soliton Dynamics in trans-Polyacetylene

Abstract

the Pariser-Parr-Pople Hamiltonian for the description of the α-electrons in trans-polyacetylene is reparametrized using ab initio Coupled Cluster Doubles calculations based on a Restricted Hartree Fock reference on trans-butadiene. To avoid the spin contaminations inherent in Unrestricted Hartree Fock (UHF) type calculations on polymethine chains in the doublet state the Annihilated Unrestricted Hartree Fock (AUHF) model is applied in our PPP calculations (tPA: (CH) ⋯ , polyenes: H-(CH) 25 -H, polymethines: H-(CH) 2N +1-H). in geometry optimizations on polymethine chains it is shown that in contrast to results from Huckel type models thewidth of neutral solitons is strongly reduced, in agreement to semiempirical all valence electron calculations (MNDO method). Further it is shown thatthe UHF model predicts artificially a metallic groundstate for polymethines due to spin contaminations, corresponding to an “infinite” soliton width. Negatively charged solitons have a larger width in agreement with MNDO and ab initio Hartree Fock results. Positively charged solitons are found to have the same width as negatively charged ones in disagreement with MNDO calculations, but in agreement with ab initio results. the photoinduced low and high energy absorptions can qualitatively be assigned to charged and neutral solitons. Within the new parametrization simulations of the dynamics of neutral polymethine chains, charged solitons, excited states, and doped chains are discussed. This work was presented at the First Congress of the international Society for theoretical Chemical Physics, June 28-July 3, 1993, inGirona.