AB-INITIO DETERMINATION OF ELECTRON MOLECULAR VIBRATION CONSTANTS (EM-V) IN BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE (BEDT-TTF) AND TETRATHIAFULVALENE (TTF)

We present results from an ab initio density functional calculation of the electron-vibronic coupling strengths in neutral and charged bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules. Results for tetrathiafulvalene (TTF) are also presented and compared with known experimental values as a measure of the accuracy of our calculation. We find that the highest occupied molecular orbital (HOMO) in ET couples strongly to modes involving the double-bonded carbon atoms as well as the sulfur atoms and that the contribution of the vibration modes involving the hydrogen atoms to the vibronic coupling is negligible. The implication of this result for the mechanism of superconductivity is discussed in the light of recent isotope effect experimental results.