COMPARATIVE APPLICATION OF DIFFERENT APPROACHES FOR BAND-STRUCTURE CALCULATIONS ON POLYPARAPHENYLENE IN THE PARISER-PARR-POPLE MODEL .2. MOLLER-PLESSET AND COUPLED-CLUSTER METHODS

We present an investigation on the calculation of the band structure o f polyparaphenylene using Moller-Plesset Perturbation Theory of 2nd or der (MPZ) to take into account correlation effects. The basis for thes e calculations is the Pariser-Parr-Pople (PPP) Hamiltonian because we also want to extract parameters from these calculations for dynamical simulations. Furthermore we want to compare different approaches to th e application of MP2 to infinite system. In the ab initio case there a re many calculations on this level published which are all based on di fferent Hartree-Fock (HF) program packages and thus use different cuto ff schemes for the interactions and different basis sets. Therefore th e results of these studies are not directly comparable and a thorough study of different approaches based on one and the same reference is h ighly desirable. In a forthcoming paper we want to present a similar s tudy on the same system using density functional schemes, especially s elf interaction corrected ones. The PPP or the Hubbard model are well suited for such a purpose, since the correlation methods on top of HF, which we outline in some detail in the first paper of this series [W. Forner, Physica Scripta 56, 490 (1997)], use exactly the same formali sms as in the corresponding ab initio applications however, the numeri cal calculations are much cheaper. Further, especially for an approach using localized orbitals we also want to discuss Coupled Cluster (CC) models, which are infinite order extensions of MP perturbation theory . We use polyparaphenylene as model system because of its importance i n diode technology. We find, that for an accuracy of the correlation c orrections in the meV region 5 to 6 neighbors are needed, while more t han 10 neighbors are required for mu eV accuracies. Further we discuss spectra calculated from our correlated band structure.