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We study here theoretically the evolution of the electronic structure of polythiophene ͑PT͒ due to bipolaron doping after modifying the -bond compressibility model. Since the electron-lattice coupling in this model alters both the site energies and the hopping integrals, effects due to the changes in the site energies on the formation of polarons and bipolarons in PT are incorporated. The inductive effect of sulfur is also considered. The ground state geometry of the neutral PT and the experimentally observed bipolaronic optical transitions are reproduced. We predict an insulating behavior of the ordered bipolaron lattice of PT. The bipolaron cluster model also fails to develop a near degeneracy of the highest occupied and the lowest unoccupied molecular orbitals at and above 20 mol % defect concentration. We attribute this to the altering of the site energies upon doping. Our model further predicts that the superlattice structure of doped PT's will be semiconductors at room temperature but with a high resistivity.

Multireference singles and doubles configuration interaction (MRDCI) calculations have been carried out to investigate the electronic structure and spectroscopic properties of the low-lying electronic states of tin oxide. A sufficiently large basis set with additional d and f polarization functions is employed. Potential energy curves of both spin-excluded and spin-included states, which correlate with the lowest dissociation limit Sn( 3 P g )+O( 3 P g ), are reported. The computed spectroscopic properties of the electronic states below 40 000 cm -1 , such as X 1 Σ + , a 3 Σ + , b 3 Π, A 1 Π, E 1 Σ + , and their spin-orbit components, are compared with the observed results. The MRDCI estimated dissociation energy of the ground state of SnO is 5.03 eV taking the d-correlation into account. Effects of the d-electron correlation on the spectroscopic constants of the ground and a few low-lying Λ-S states have been explored. Transition probabilities of a few transitions are estimated. The A 1 Π-X and E 1 Σ + -X transitions are predicted to be quite strong. The spin-forbidden transitions such as b 3 Π 1 -X and b 3 Π 0 + -X are compared with the Cameron band of the isovalent CO molecule. The calculated dipole moments of SnO as a function of the bond length in three low-lying states, namely X 1 Σ + , A 1 Π, and E 1 Σ + , have been reported.