The electronic structure and the coupling of electrons to the vibrational modes of a single bis ͑ethylenedithio͒-tetrathiafulvalene ͑BEDT-TTF͒ molecule, the building block of a class of organic superconductors, are studied from quantum chemical calculations. We find that the molecular orbital structure near the Fermi energy can be described with a simple tight-binding model involving only the out-of-plane p orbitals on the carbon and sulfur atoms in the central part of the molecule. Just three of the vibrational modes of A g symmetry couple strongly to the electron states resulting in a frequency shift of typically 10% for these modes. The high-frequency A g (11) mode is predicted to be broadened the most by the coupling, by an amount of ϳ8%. The calculated electron-phonon coupling constant in the solid, ϳ0.4, is strong enough to produce a superconducting transition temperature T c ϳ3 -10 K as estimated from McMillan's formula, indicating that the superconductivity in the BEDT-TTF compounds can be explained within the standard BCS electron-phonon coupling theory.

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Ab initio electronic structure calculations have been performed for the van der Waals dimer HCN⋅SO2 and the associated monomers, HCN and SO2. The results of both Hartree–Fock and Mo/ller–Plesset calculations are reported for energy, optimized geometry, rotational constants, and vibrational harmonic frequencies for each system. An anti-hydrogen-bonded structure is predicted to be the most stable structure for the dimer in agreement with a previous experimental analysis.

Abstract. Electronic structure calculations employing ab initio methods have been carried out for the H2S monomer and two of its associated dimers: its homodimer, (H2S)2 and its heterodimer with water. The results reported here include geometric parameters, the zero-point and minimum energies, rotational and vibrational constants, and dipole moments. The contribution of the monomers to the dimer spectrum has been suggested. Two structures for the homodimer corresponding to potential surface stationary points have been found. One of these has been identified as a transition state.