From organic superconductors to DNA: Fragment orbital‐based model

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AbstractA semi‐empirical valence bond/Hartree–Fock (VB/HF) method is developed to calculate one‐ and two‐electron interactions between molecular fragments in conducting supramolecular stacks. This fragment orbital‐based formalism allows for consistent extraction of an effective Hamiltonian defined as a “frontier orbital” model. This Hamiltonian quantitatively describes transfer and electrostatic interactions between conducting electrons, while reducing the active space so dramatically that the electronic eigenstates of very large systems may be investigated. The capabilities of the VB/HF method are illustrated on two different supramolecular stacks involving a π–π interacting fragment. In the first part of this study, the framework of the VB/HF method is used to evaluate the relative magnitude of the electronic interactions between conduction electrons in organic conductors and superconductors derived from Bechgaard salts. In the second part of this study, the VB/HF formalism is extended to derive an effective model for conduction holes along doped DNA double strands. Transferable intra‐ and intersite parameters were first evaluated from VB/HF calculations carried out on nucleoside pairs. From this interaction databank, the effective Hamiltonian of any type of nucleoside sequence can be defined. The thermalized charge distribution for a single hole delocalized along a DNA sequence containing 240 Watson–Crick pairs is then calculated and compared with the experimental yields of damage revealed by photocleavage experiments. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006