QMMM Chemical Simulations

A recently developed method to describe ions in solution by combined Born-Oppenheimer Quantum Mechanics - Molecular Mechanics simulations of either Molecular Dynamics or Monte Carlo type will be implemented using parallel processing for the most time-consuming computational part, namely the evaluation of Hartree-Fock forces and energies, thus allowing the investigation of a series of solvated cations in water, pure and aqueous ammonia. The results of the first applications of QM/MM simulations have not only shown that the implicit inclusion of many-body effects (up to 10) improves decisively the results for solvated divalent ions [1,2], but appears also necessary for the correct reproduction of fundamental properties of monovalent main group cations as structure- forming and structure-breaking effects [3]. The well-known Jahn-Teller effect for Cu(II) in aqueous solution could also be reproduced and investigated in detail only on the basis of this multibody, quantum mechanically corrected approach [2]. The focus of the applications envisaged for this project will be centered on simulations of divalent and trivalent first row transition metal ions, which are not only of fundamental interest for the understanding of their complex chemistry (in both senses of this term!), but will also provide crucial test examples for widely used chemical models such as crystal field and ligand field "theory". The new approach is expected, by the inclusion of many- body effects for the complete first solvation shell, to give the most accurate description available so far for these ions in solution, and to supply substantial data for the interpretation of ligand exchange mechanisms for these ions and thus for testing also the exchange-related simple models widely used in the discussion of spectroscopic data, e.g. high-pressure NMR experiments. In the case of aqueous ammonia solutions of varying composition, the coordination number distributions available from a detailed analysis of the simulations will further allow a comparison with complex formation constants determined experimentally, and supply detailed structures for all microspecies realised in the equilibrium state of such a solute-solvent system.

Using Molecular Dynamics Simulations a highly accurate description of structure and dynamics was obtained for a large number of metal ions in aqueous solution. The high accuracy was achieved by a complete quantum mechanical treatment of the ions and their nearest environment, and the enormous computational effort of several months of CP time per ion could be managed in a most economic way using parallel-processing clusters of PCs. The data obtained do not only represent a substantial progress in understanding the chemistry of electrolyte solutions, but also for numerous biological processes, where the investigated main group and transition metal ions play a central role. In order to visualize the results, a new program package has been developed, which allowed the production of video clips showing the dynamical processes on a femto and pico second scale, thus allowing insight into important reactions such as the exchange process of ligands. The results lead to 20 publications in international journals, 2 invited review articles and 2 invitations for plenary lectures at international conferences.