Accuracy of buffered-force QM/MM simulations of silica. Peguiron, A., Ciacchi, L. C., Vita, A. D., Kermode, J. R., & Moras, G. Journal of Chemical Physics, American Institute of Physics, February, 2015.
Accuracy of buffered-force QM/MM simulations of silica [link]Paper  abstract   bibtex   
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 $∼$5 Å previously reported for covalent materials such as silicon. Electrostatic embedding of the QM region in the surrounding classical point charges gives only a minor contribution to the force convergence. While the energy-based approach provides accurate results in geometry optimizations of point defects, we find that the removal of large force errors at the QM/MM boundary provided by the buffered force-based scheme is necessary for accurate constrained geometry optimizations where Si?O bonds are elongated and for finite-temperature molecular dynamics simulations of crack propagation. Moreover, the buffered approach allows for more flexibility, since special-purpose QM/MM coupling terms that link QM and MM atoms are not required and the region that is treated at the QM level can be adaptively redefined during the course of a dynamical simulation.
@article{wrap66305,
          volume = {142},
           month = {February},
           title = {Accuracy of buffered-force QM/MM simulations of silica},
          author = {Anke Peguiron and Lucio Colombi Ciacchi and Alessandro De Vita and James R. Kermode and Gianpietro Moras},
       publisher = {American Institute of Physics},
            year = {2015},
         journal = {Journal of Chemical Physics},
             url = {https://wrap.warwick.ac.uk/66305/},
        abstract = {We report comparisons between energy-based quantum mechanics/molecular mechanics (QM/MM) and buffered force-based QM/MM simulations in silica. Local quantities{--}such as density of states, charges, forces, and geometries{--}calculated with both QM/MM approaches are compared to the results of full QM simulations. We find the length scale over which forces computed using a finite QM region converge to reference values obtained in full quantum-mechanical calculations is {$\sim$}10 {\rA} rather than the {$\sim$}5 {\rA} 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.}
}

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