聚乙烯模型搭建一些问题求助

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老师们好，我在重复文献模型过程中，遇到了一些问题。文献中作者创建如图一的模型，固定上下层金刚石，然后在NVE系综下升温至600k然后保持一段时间.退火的时候上层金刚石往下移动，最后挪去上层，变成图二的模型。请问老师们，这个地方如何做到在退火过程中使得上层原子往下移动的呢？还有就是他的系综一直用的NVE吗？英文部分为原文，希望老师们能给予帮助。To get a more homogeneous
configuration and smooth surface for each substrate, an annealing simulation within NVE ensemble for each cell was performed.The substrate atoms are Newtonian atoms, whose motions obey classic Newton's second law. The Newton's equation of motion was integrated by velocity-Verlet algorithm with a time step of 0.5 fs in both cases.Periodic boundary conditions were imposed in the 3 directions and a rescaling algorithm was set to adjust the temperature via modifying the atom velocities. The PCFF force field was employed to describe the interactions between atoms. In PE case, after fixing the top layer and bottom layer, the PE cell was heated gradually to 600 K and kept at 600 K for 100 ps, allowing the solid to melt. The substrate was then annealed at a constant rate over 100 ps to 200 K, which is lower than the glass-transition temperature (Tg) of PE. During the annealing process,the top layer was moved toward –z direction as a rigid body at a constant rate to compress the substrate volume and ultimately allow the substrate density reach about 0.95 g/cm3.