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.

1. M. Neumann, M. R. Jonhson, J. Chem. Phys. 107, 1725 (1997).
2. M. Neumann, G. J. Kearley, Chem. Phys., 215 253 (1997)
3. G. J. . Kearley, J. Tomkinson, A. Navarro,J. J. Lopez Gonzalez, M. Fernandez Gomez, Chem. Phys., 216 323 (1997).
4. P. Schiebel, G. J. Kearley, M. R. Johnson, J. Chem. Phys., 108 2375 (1998)