title: Prediction of molecular crystal structures by a crystallographic QM/MM model with full space-group symmetry.
abstract: A crystallographic quantum-mechanical/molecular-mechanical model (c-QM/MM model) with full space-group symmetry has been developed for molecular crystals. The lattice energy was calculated by quantum-mechanical methods for short-range interactions and force-field methods for long-range interactions. The quantum-mechanical calculations covered the interactions within the molecule and the interactions of a reference molecule with each of the surrounding 12-15 molecules. The interactions with all other molecules were treated by force-field methods. In each optimization step the energies in the QM and MM shells were calculated separately as single-point energies; after adding both energy contributions, the crystal structure (including the lattice parameters) was optimized accordingly. The space-group symmetry was maintained throughout. Crystal structures with more than one molecule per asymmetric unit, e.g. structures with Z' = 2, hydrates and solvates, have been optimized as well. Test calculations with different quantum-mechanical methods on nine small organic molecules revealed that the density functional theory methods with dispersion correction using the B97-D functional with 6-31G* basis set in combination with the DREIDING force field reproduced the experimental crystal structures with good accuracy. Subsequently the c-QM/MM method was applied to nine compounds from the CCDC blind tests resulting in good energy rankings and excellent geometric accuracies.
authors.affiliation: Institut für Anorganische und Analytische Chemie, Goethe-Universität, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Mörschel
authors.affiliation: Institut für Anorganische und Analytische Chemie, Goethe-Universität, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.
authors.forename: Martin U
authors.initials: MU
authors.lastname: Schmidt
keywords: blind test; crystal structure prediction; density functional theory; quantum mechanics/molecular mechanics