**International Journal of Quantum Chemistry**

*Ab initio* coupled and uncoupled Hartree–Fock calculations of the polarizabilities of finite and infinite polyacetylene chains

## Abstract

In this work, *ab initio* coupled and uncoupled Hartree-Fock polarizability calculations are carried out on finite and infinite polyacetylene chain models. We describe the polymeric procedures which are extensions to infinite systems of the molecular Sum Over States (SOS) and Random Phase Approximation (RPA) methods. The latter does include the field-induced electron reorganizational effects whereas the former does not. The inclusion of these effects is particularly important in computing the longitudinal component of the dipole polarizability tensor of polyacetylene chains at an accurate level since the ratio of the coupled/uncoupled results is between 2 and 4 for the conjugated systems studied. In order to evaluate the asymptotic longitudinal polarizabilities per unit cell we have used both finite oligomeric calculations followed by extrapolation procedures and direct polymeric methods. The difficulty of using the extrapolation procedures due to the unknown analytical form of the evolution of the longitudinal polarizability per unit cell illustrates the convenience of using our direct method treating infinite polymeric systems. We obtain a good agreement between our infinite chain results and the largest oligomeric values. We also discuss the choice of atomic basis set, the possibility of using scaling factors, and the crucial role of using a correct bond length alternation value. © 1993 John Wiley & Sons, Inc.