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.

On modelling disordered crystal structures through restraints from molecule-in-cluster computations, and distinguishing static and dynamic disorder

“…Other applications were centred on computing geometries (Bjornsson & Bü hl, 2012) and bond distances involving hydrogen atoms (Dittrich et al, 2017), which can be extended to excited-state conformations of a molecule in the solid (Kamiń ski et al, 2010). Even polymorph energy ranking was carried out (Mö rschel & Schmidt, 2015). All these computations have in common that a cluster, which consists of molecules or ions in proximity to a central system, reproduces the crystal environment.…”

Structure determination of organic compounds by a fit to the pair distribution function from scratch without prior indexing

“…If indexing fails, potential lattice parameters and possible space group(s) can be obtained, e.g. by a time-consuming crystal structure prediction (Bardwell et al, 2011;Neumann et al, 2015), although the comparison of the simulated powder patterns of the predicted crystal structures with the experimental powder pattern can only lead to the average crystal structure (Mö rschel & Schmidt, 2015).…”

A Full Additive QM/MM Scheme for the Computation of Molecular Crystals with Extension to Many-Body Expansions

“…So far, no analytic gradients are available. The crystal-QM/MM model of Mörschel and Schmidt 36 uses point charges and parametrised 6-exp potentials, but does only allow for an optimisation of the cell parameters. Finally, Bjornsson and Bühl 37 presented a basis for additive QM/MM with a fixed environment, which we discuss in greater detail in the Method Section.…”