The CNDO/S3 crystal orbital model: Definition and application to polyacetylene

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Abstract
The CNDO/S3 model, a spectroscopically parametrized complete neglect of differential overlap molecular orbital method, is extended to encompass the description of radical cation states in infinite periodic quasi‐one‐dimensional molecules. Detailed model calculations of the energy bands and density of valence electron states (DOVS) are given for macromolecules of trans polyacetylene. These calculated quantities are shown to correspond directly to predictions extrapolated from sequences of CNDO/S3 calculations for finite polyenes, and to describe accurately both results of ab initio calculations and measured valence electron photoemissionspectra for polyacetylene. The consequences of introducing electron–electron interactions into a noninteracting electron model are studied systematically by examining in turn the Hückel, Hubbard, CNDO–SCF, and ab initio limits of the calculated energy bands and DOVS. Comparison of the results of these calculations with measured photoemissionspectra for polyacetylene reveals that electron–electron interactions exert a minor effect on the interpretation of these spectra. Since the CNDO/S3 model has previously been shown to provide a quantitative description of the valence electron photoemission of (gas‐phase) finite polyenes, the present analysis of macromolecules of trans polyacetylene provides an explicit link between the interpretation of these spectra for individual finite polyene molecules and the analysis of the corresponding spectra obtained from films of polyacetylene.