Correlation corrected energy bands of poly(para-phenylene-vinylene)

The band structure of poly( para -phenylene-vinylene) (PPV) has been calculated. A Hartree–Fock (HF) crystal orbital calculation served as a starting point for a quasi-particle (QP) method to take the electron correlation into account. A double zeta basis set was used with polarization functions. The calculated ionization potential (using Koopmans' theorem) is somewhat larger than the experimental one and the width of the correlation corrected valence band is in quite good agreement with the value obtained from previous calculations. On the other hand, although the correlation correction decreased the fundamental gap substantially as compared with its HF value (from 7.42 to 4.89 eV), it is still twice as large as its directly measured value. The main reason for this large disagreement is that we have calculated a single chain of PPV, while the measured value refers to a layer. Looking at the crystal structure of PPV one finds that it is closely packed with the smallest interchain distance of ∼2.5 A. In such case no Madelung potential can be used to describe the rest of the crystal around the calculated chain. For this reason we plan different two-dimensional calculations to resolve the contradiction concerning the fundamental gap.