Toward a Practical Method for Adaptive QM/MM Simulations.

We present an accurate adaptive multiscale molecular dynamics method that will enable the detailed study of large molecular systems that mimic experiment. The method treats the reactive regions at the quantum mechanical level and the inactive environment regions at lower levels of accuracy, while at the same time molecules are allowed to flow across the border between active and environment regions. Among many other things, this scheme affords accurate investigation of chemical reactions in solution. A scheme like this ideally fulfills the key criteria applicable to all molecular dynamics simulations: energy conservation and computational efficiency. Approaches that fulfill both criteria can, however, result in complicated potential energy surfaces, creating rapid energy changes when the border between regions is crossed. With the difference-based adaptive solvation potential, a simple approach is introduced that meets the above requirements and reduces fast fluctuations in the potential to a minimum. In cases where none of the current adaptive QM/MM potentials are able to properly describe the system under investigation, we use a continuous force scheme instead, which, while no longer energy conserving, still retains a related conserved quantity along the trajectory. We show that this scheme does not introduce a significant temperature drift on time scales feasible for QM/MM simulations.