On the structure dependence of the static longitudinal linear electric polarizability of infinite polyenes: An ab-initio uncoupled perturbed Hartree-Fock crystal orbital study

Abstract

The static longitudinal linear electric polarizability, α, has been computed for several values of the bond alternation parameter, Δr, for infinite polyene (trans-polyacetylene) chains using the ab initio uncoupled perturbed Hartree-Fock (UPHF) crystal orbital method with different atomic basis sets. The computed values of α(UPHF) were found to increase by a factor of 5–6 with decreasing bond alternation, exhibiting typical values of about 2–300 a.u. in the neighborhood of Δr optimized for alternating (trans-polyacetylene). Cluster calculations have also been performed for the polyene series H−(C₂H₂) _n −H, n=3–10, using both the UPHF and the coupled perturbed Hartree-Fock (CPHF) methods, respectively. Since the geometrical skeleton of these clusters was built from the structural parameters of the corresponding infinite polyacetylenes, their properties, as extrapolated to the limit n→∞, provided a possibility for comparing the performance of the UPHF and CPHF methods, respectively, in the case of the infinite systems. It turned out that the α(UPHF) values, which amount to about 80% of the corresponding α(CPHF) results for shorter polyenes, recover less than 40% of the polarization for infinite chains, pointing thus to the necessity of an improved description of electron rearrangement processes in polymers.