Charged trans polyacetylene: a density-functional study

Abstract

In this paper we study the electronic properties of a single doped chain of trans polyacetylene. First, we discuss the different theoretical approaches for studying infinite, periodic, charged chains within parameter-free, electronic-structure calculations. Subsequently, one of these is applied for trans polyacetylene using a density-functional-based method. In particular, the bond-length alternation and the energy gain due to this alternation as a function of added charge are studied. Thereby, it is demonstrated that trans polyacetylene is lacking an electron–hole symmetry. We also explore whether the density-functional calculations predict a too large bond-length alternation when a finite set of points is used in the k-space sampling. Finally, the first-principles results are analysed within a many-body Hubbard-like model Hamiltonian. We find that when extra charge is added to every cell the resulting Hubbard parameters are considerably larger than the ones usually attributed to trans polyacetylene and closer in value to those of the isolated atom. We discuss this result in relation to the importance of screening effects and structural relaxations.