Abstract
For pt.III see Solid State Commun., vol.29, p.251 (1979). The electronic ground state of infinite polydiacetylene chains has been investigated with the aid of the crystal orbital method. For both the acetylenic and the butatrienic arrangement the total energy was optimised as a function of interatomic distances and bond angles. The acetylenic structure turned out to be stabilised over the butatrienic one by approximately=12 kcal mol-1 per C4H2 unit. The respective equilibrium geometries and a selected number of force constants are reported as well. The electronic band structure of both isomeric structures is analysed and differences in the occupation schemes are discussed.