Towards an understanding of the vibrational mode specificity for dissociative chemisorption of CH4 on Ni(111): a 15 dimensional study

We present a fifteen-dimensional (15D) vibrational mode following calculation along the minimum energy path (MEP) of the dissociative chemisorption of CH4 on Ni(111), based on density functional theory (DFT). The MEP has been obtained from the (climbing image) nudged elastic band and steepest descent methods employing a periodic DFT code. The MEP displays a late barrier of 1.09 eV, which is reduced to 0.95 eV after zero-point energy corrections, and a considerable CH bond elongation at the transition state, in accordance with earlier calculations. Our vibrational calculations within the harmonic approximation show a significant involvement of the different vibrational coordinates in reaching the transition state. The couplings between the normal modes have been analyzed along the MEP, various crossing and avoided crossing regions have been identified and discussed in connection with the corresponding Massey velocities and parameters. Based on our analysis we find that pre-exciting the symmetric stretch vibration should be approximately 3 times as efficient in promoting reaction as pre-exciting the asymmetric stretch vibration. The analysis also suggests that the bend vibrations do not couple well to the reaction coordinate, whereas the stretches are efficiently coupled to the reaction coordinate. Both results are in good agreement with experiments.