A new formulation of ab initio theory is presented that treats a large molecule in terms of wavefunctions of its constituent molecular subunits (to be called fragments). The method aims to achieve near conventional ab initio accuracy but using a truncated set of fragment orbitals with a consequent drastic reduction of computing time and storage requirement.
Both theoretical and experimental studies in the past have indicated that the charge transport in lightly doped polyacetylene is due primarily to traveling charged solitonic waves along the polymer chain backbone accompanied by hopping from one chain to another. The conductivity in this model is still dictated by a bandgap. The nature of the ground and excited states of the doped system, however, is not fully understood. Previous ab initio calculations on polyenes doped by a single iodine atom have brought out the interesting feature that, while calculations at the Hartree- Fock level lead to the charge-transfer state as the ground state, a correlated calculation, on the other hand, shows it to be an excited state with the ground state showing little charge transfer. Since, however, only polyiodide anions I3-, I5-, etc. are found in solution rather than neutral radicals such as I, I3 etc., inferences based on the calculation employing a single iodine atom are not conclusive. We present here a systematic ab initio study in which the nature of the ground and excited states of polyenes, doped with iodine, are investigated.