Examples of LDA Calculations on Polymers

The band structures are quite similar in the GKS and PZ cases. The calculated gaps (7.7 eV for GKS and 8.0 eV for PZ) are in a rather good agreement with the value of 8.8 eV estimated on the basis of experiment.97 This result is somewhat surprising, because LDF calculations usually underestimate the gap in semiconductors by 20-50%. 

The calculated gaps of the band structures are 5.5 and 5.8 eV at the most stable conformation. These values are somewhat smaller than the one obtained by Kasowski et a/.80 using a linear combination of muffin tin orbitals in their LDA calculation. It should be mentioned that these values are still essentially smaller than the gap of 7.2 eV which we have obtained for our quasi particle band structure at the MP2 level.67 One should observe also that the lower limit of the conduction band is essentially higher (—1.5-2.0 eV), than in the HF + MP2 calculation ( — 5.7 eV67). Being, however, negative in both approximations (GKS and PZ, respectively) it still indicates a possibility of n-doping. 

Finally an LDA calculation was also performed for a polyparaphenylene (PPP) helix.100 This conducting polymer (if doped) is quite interesting for its applications in light emitting diodes101 (especially because it emits blue light102), in field-effect transistors103 and in non-linear optical devices.104 

A geometry optimization has provided an intra-ring CC distance (no distinction has been made between the two possible CC interring distances) l = 1.39 A, an inter-ring CC distance of 1.47 A, a CH distance of 1.10 A and a valence angle of 121°. These values100 are within the range of the different experimental data.105 

88 eV73 and is close to the probably underestimated experimental value (on 

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