DENSITY-FUNCTIONAL STUDY OF CIS-1,4-POLYBUTADIENE AND TRANS-1,4-POLYBUTADIENE

Results of density-functional calculations on cis-and trans-1,4-polybu tadiene are presented. The resulting band structures are compared with those of polyethylene and trans-polyacetylene. Charge-density plots o f the highest occupied rt band show that the results are in accordance with the intuitive concept of localized pi bonds. Tight-binding fits of the band structure show that polybutadiene can be regarded as a cop olymer between polyethylene and polyacetylene. A detailed study of the single-particle energies of polybutadiene, polyethylene, polyacetylen e, and ethylene allows finally a determination of some of the paramete rs for an extended Hubbard model that can describe the rr electrons of extended hydrocarbons. It is shown that the nearest-neighbor Hubbard parameters are small, indicating that further interactions are effecti vely screened.