Electronic structures of polyethylene and polytetrafluoroethylene

Results of first-principles calculations on polyethylene [poly-(${\mathrm{CH}}_2$)] and polytetrafluoroethylene [poly-(${\mathrm{CF}}_2$)] are reported. We have optimized the carbon-carbon bond lengths for polyethylene, and using the optimized value we have considered structures in which the carbon backbone is assumed helical both for polyethylene and for polytetrafluoroethylene. The calculated bond lengths and valence-band structures are compared with those of other theoretical approaches and with experimental data. It is finally discussed why polyethylene has a planar carbon backbone whereas polytetrafluoroethylene is nonplanar.