Charged trans polyacetylene: a density-functional study

In this paper we study the electronic properties of a single doped chain of trans polyacetylene. First, we discuss the different theoretical approache s for studying infinite, periodic, charged chains within parameter-free, el ectronic-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 t rans polyacetylene is lacking an electron-hole symmetry. We also;explore wh ether the density-functional calculations predict a too large bond-length a lternation when a finite set of points is used in the k-space sampling. Fin ally, the first-principles results are analysed within a many-body Hubbard- like model Hamiltonian. We find that when extra charge is added to every ce ll the resulting Hubbard parameters are considerably larger than the ones u sually attributed to trans polyacetylene and closer in value to those of th e isolated atom. We discuss this result in relation to the importance of sc reening effects and structural relaxations. (C) 2000 Elsevier Science B.V. All rights reserved.