Solids receive displacement damages (DD) when interacting with energetic particles, which may happen during the fabrication of semiconductor devices, in harsh environments and in certain analysis techniques. Simulations of the DD generation are usually carried out with classical molecular dynamics (MD), but classical MD does not account for all the effects in DD, as demonstrated by <i>ab initio</i> calculations of model systems in literature. A fully <i>ab initio</i> simulation of the DD generation is impractical due to the large number of atoms involved. In my previous paper [Phys. Chem. Chem. Phys. 22, 19307 (2020)], I developed an adaptive-center (AC) method for adaptive-partitioning (AP) quantum mechanics/molecular mechanics (QM/MM) simulations, allowing the active region centers and the QM/MM partition to be determined on-the-fly for energy-conserving AP-QM/MM methods. I demonstrated that the AC-AP-QM/MM is applicable to the simulation of the DD generation, so that the active regions can be treated with an <i>ab initio</i> method. The AC method was unable to identify the fast-moving recoil ions in the DD generation as active region centers, however, and the accuracy is negatively affected by the rapid change in QM/MM partition of the system. In this paper, I extend the AC method and develop a speed-dependent adaptive-center (SDAC) method for proper AP-QM/MM simulations of DD. The SDAC method is applicable to general problems with speed-dependent active regions, and is compatible with all existing energy-conserving partition-by-distance AP-QM/MM methods. The artifact due to the speed-dependent potential energy surface can be made small by choosing proper criteria. I demonstrate the SDAC method by simulations of the DD generation in bulk Silicon.<br>
On-the-Fly Determination of Active Region Centers in Adaptive-Partitioning QM/MM