title: Accuracy of buffered-force QM/MM simulations of silica.
abstract: We report comparisons between energy-based quantum mechanics/molecular mechanics (QM/MM) and buffered force-based QM/MM simulations in silica. Local quantities-such as density of states, charges, forces, and geometries-calculated with both QM/MM approaches are compared to the results of full QM simulations. We find the length scale over which forces computed using a finite QM region converge to reference values obtained in full quantum-mechanical calculations is ∼10 Å rather than the ∼5 Å previously reported for covalent materials such as silicon. Electrostatic embedding of the QM region in the surrounding classical point charges gives only a minor contribution to the force convergence. While the energy-based approach provides accurate results in geometry optimizations of point defects, we find that the removal of large force errors at the QM/MM boundary provided by the buffered force-based scheme is necessary for accurate constrained geometry optimizations where Si-O bonds are elongated and for finite-temperature molecular dynamics simulations of crack propagation. Moreover, the buffered approach allows for more flexibility, since special-purpose QM/MM coupling terms that link QM and MM atoms are not required and the region that is treated at the QM level can be adaptively redefined during the course of a dynamical simulation.
authors: Fraunhofer IWM, Wöhlerstraße 11, 79108 Freiburg, Germany.
authors: Anke
authors: A
authors: Peguiron
authors: Hybrid Materials Interfaces Group, Faculty of Production Engineering and Bremen Center for Computational Materials Science, University of Bremen, Am Fallturm 1, 28359 Bremen, Germany.
authors: Lucio
authors: L
authors: Colombi Ciacchi
authors: Department of Physics, King's College London, Strand, London WC2R 2LS, United Kingdom.
authors: Alessandro
authors: A
authors: De Vita
authors: Department of Physics, King's College London, Strand, London WC2R 2LS, United Kingdom.
authors: James R
authors: JR
authors: Kermode
authors: Fraunhofer IWM, Wöhlerstraße 11, 79108 Freiburg, Germany.
authors: Gianpietro
authors: G
authors: Moras