  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.
