QUANTUMCRYSCA: a new approach for the prediction of molecular crystal structures by a crystallographic QM-MM shell model
Philipp Mörschel, M.U. Schmidt · Acta Crystallographica Section A Foundations of Crystallography · 2012
The ab initio treatment of molecular crystals in due consideration of space group symmetry is still an underdeveloped field.Quantumcrysca is an innovative attempt to calculate the energy in periodical systems by quantum-mechanical means including full treatment of space group symmetry using a QM-MM-approach.Dividing the crystal in disjunct spherical regions gives rise to a shell-model.All interactions between atoms in the inner sphere are treated on a quantum mechanical level, whereas the interactions in the outer sphere and between the inner and the outer sphere are calculated by classical force-fields.A reliable quantum-mechanical calculation of weak long-range interactions (London dispersion, van der Waals-interaction) in periodical systems turns out to be especially challenging.Correlation methods include this kind of interaction but are computationally demanding.Pure density-functional theory neglects long-range interactions.A good compromise can be found in dispersion-corrected density-functional theory (DFT-D).1,2 First efforts have been made by Neumann et.al. using plane wave DFT-D in periodical systems.3 In Quantumcrysca the quantum-mechanical sphere is efficiently described using localised orbitals (Gaussian type orbital, GTO), leading to a massive saving of computing time and a more intuitive view of the electronic state using molecular orbitals.In Quantumcrysca various local optimization algorithms using analytical derivatives are implemented, including steepest-descent, conjugate-gradient and quasi-Newton-methods. Faster convergence is achieved by additional linesearch in each optimization step.The combination of these local optimization techniques with a structure-generation algorithm can be used to perform a complete crystal structure prediction in the absence of experimental data.Quantumcrysca is designed for the validation of crystal structures as well as for the performance of a full crystal structure prediction.First results will be presented.