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Abstract

Results of first-principles, density-functional, LMTO calculations on quasi-one-dimensional systems are presented. Special emphasis is put on the problems related with studying the properties of real materials by means of electronic structure calculations. As examples we discuss undoped and doped trans polyacetylene, an MX chain compound, CaNiN, and a comparison between polycarbonitrile and hydrogen cyanide. It is argued that single-chain calculations on undoped polyacetylene predict a bond-length alternation, whereas this may be suppressed in multichain calculations. For an MX chain we demonstrate the importance of including the full ligand structure, whereas many properties of CaNiN can be understood as intrinsic to single chains. A comparison between the covalently bonded polycarbonitrile and the hydrogen-bonded hydrogen cyanide reveals differences between delocalized and localized electrons but also inaccuracies in the relative total energies. Finally, results for doped trans polyacetylene indicate relatively large electronic dopant-polymer hopping integrals as well as problems in describing electron transfers for weakly interacting systems when using a local-density approximation. In total it is demonstrated that reliable and useful informations are obtained in most cases, but also that one has to be careful in choosing the idealized system that is to represent the real material and that certain aspects of the local-density approximation should be improved. © 1993 John Wiley & Sons, Inc.