Error analysis and applications of a general QM/MM approach

Combing computational efficiency and accuracy, quantum mechanic/molecular mechanic (QM/MM) methods are promising for understanding and predicting materials properties at atomic scales. We present a general QM/MM method that can be implemented in a variety of QM and MM combinations; we focus on a tight-binding and a plane-wave pseudopotential methods for the QM part and the Stillinger-Weber and EAM potentials for the MM part. The QM/MM coupling errors are analyzed. It is found that a sufficient size of the coupling region and the buffer zone is crucial in minimizing the coupling errors. On the other hand, the size of the weighting zone turns out to be less important. The QM/MM method is applied to model the dynamical propagation of Si cracks with different orientations and under different mode I loadings. The QM/MM method is found to correctly reproduce the brittle fracture of Si, whereas the SW potential fails to do the same. The QM/MM method is also used to study the ductile fracture in Au and compared to the EAM potential. Finally, the QM/MM method is applied study the vacancy diffusion in a Cu grain boundary. The QM/MM results compare very well to the previous EAM results.