COMBINED USE OF COMPUTATIONAL CHEMISTRY AND NEUTRON SCATTERING: STRUCTURE, ROTATIONAL TUNNELLING AND MOLECULAR VIBRATION
M.R. Johnson and G.J. Kearley
Institut Laue Langevin, BP156 38042, Grenoble, Cedex 09, France.
Whereas time-dependent classical dynamics can be modelled with molecular-dynamics techniques, quantum dynamics can only be reproduced by calculating the potential-energy surface and resolving the Hamiltonian corresponding to the molecular motion of interest. We have used time-independent techniques of this type to study small-molecule crystalline systems using a wide range of numerical methods, from force fields to quantum chemistry, to the most accurate experimental data, namely tunnelling and vibrational spectroscopy. From an experimental viewpoint, this approach enables a quantitative, imodel-freeî analysis of quantum dynamics, in particular the subtle interplay between structure and dynamics.
Starting from the time-average structure determined at the same temperature as the spectroscopic measurements we have been able to account for the rotational tunnelling spectra of methyl[1] and ammonium systems, INS vibrational spectra of small organic molecules [2,3]. In some cases it has been possible to compare measured nuclear density with that calculated from the wavefunctions of the quantum system [4]. Based on these combined results we will discuss the relative merits of: force-field, semi-empirical, Hartree-Fock SCF, and DFT methods.