A SIMPLIFIED PICTURE FOR PI-ELECTRONS IN CONJUGATED POLYMERS - FROM PPP HAMILTONIAN TO AN EFFECTIVE MOLECULAR-CRYSTAL APPROACH

An excitonic method appropriate to study conjugated oligomers and poly mers is described and its applicability tested on the ground and first excited states of trans-polyacetylene, taken as a model. From the Par iser-Parr-Pople Hamiltonian, we derive rut effective Hamiltonian based on a local description of the polymer in terms of monomers: the relev ant electronic configurations are built on a small number of pertinent local excitations. The intuitive and simple microscopic physical pict ure given by our model supplements recent results, such as the Rice an d Garstein ones. Depending on the parameters, the linear absorption ap pears dominated by an intense excitonic peak.