Extrapolation Methods for Improving the Convergence of Oligomer   Calculations to the Infinite Chain Limit of Quasi-Onedimensional   Stereoregular Polymers
  Quasi-onedimensional stereoregular polymers as for example polyacetylene are currently of considerable interest. There are basically two different approaches for doing electronic structure calculations: One method is essentially based on concepts of solid state theory. The other method is essentially a quantum chemical method since it approximates the polymer by oligomers consisting of a finite number of monomer units. In this way, the highly developed technology of quantum chemical molecular programs can be used. Unfortunately, oligomers of finite size are not necessarily able to model those features of a polymer which crucially depend of its in principle infinite extension. In such a case extrapolation techniques can be extremely helpful. For example, one can perform electronic structure calculations for a sequence of oligomers with an increasing number of monomer units. In the next step, one then can try to determine the limit of this sequence for an oligomer of infinite length with the help of suitable extrapolation methods. Several different extrapolation methods are discussed which are able to accomplish an extrapolation of energies and properties of oligomers to the infinite chain limit. Calculations for the ground state energy of polyacetylene are presented which demonstrate the practical usefulness of extrapolation methods. 
Ernst Joachim Weniger (Institute for Physical and Theoretical   Chemistry, University of Regensburg, Regensburg, Germany), B...