Graded methods for rapid generation of quantum mechanical forces in molecular dynamics simulations

Multi-scale simulations in materials and biomolecular systems are bottlenecked by the quantum mechanical calculation of forces in a chemically active region. As an alternative strategy to the relatively drastic simplifications involved in tight-binding or order-N QM methods, we suggest a sequence of QM and classical approximations graded by accuracy, hence computational cost. The strategy is to use the more costly but more accurate approximations at relatively infrequent simulation steps to reset the forces from the faster approximations of lower-grade accuracy. We illustrate with a severe test, comprised of only two grades, namely a published classical pair potential and a QM method independently calibrated to reproduce relevant coupled-cluster forces.