Research On Multiscale Methods Based On Smoothed Molecular Dynamic

Numerical simulation is one of most important research tools in the micro/nano field,and molecular dynamics(MD)is widely used in micro/nano-scale simulation.However the temporal and spatial scales in MD simulation is limited,due to huge computational cost and limited computing power.As a recently-proposed efficient molecular simulation method,smoothed molecular dynamics(SMD)allowed much larger time step size than MD.However,as high-frequency motions are smoothed,local atom disorders are not well described in SMD.By combining methods of different scales,multiscale methods achieve a balance between efficiency and accuracy.This research attempts to improve SMD or develop new multiscale method based on SMD.An improvement of SMD method,called AltSMD method,is proposed by alternating with MD relaxation process.AltSMD method can be understood as a generalized prediction-correction algorithm,in which SMD computation is used throughout the whole solution,and it is converted to MD relaxation whenever required.The accuracy can be guaranteed by MD relaxations,and the overall efficiency be guaranteed by large time step in SMD.A criterion based on the difference of MD forces and SMD forces is proposed to realize automatically alternating process and then further improve the efficiency.High-frequency reflection occurs due to different size in fine region and coarse region.A transition scheme based on frequency decomposition is proposed,to eliminate spurious wave reflection at the interface in multiscale methods.The new scheme adopt a transition region surrounding the interface,where damping force is applied only on highfrequency components.Thus high-frequency wave is absorbed in transition region,while low-frequency wave is little affected.The mapping process in SMD is utilized to implement frequency decomposition efficiently.The transition scheme is simple in formation and can be applied to various multiscale methods.To make the best of high accuracy in MD and high efficiency in SMD,MD-SMD adaptive coupling method and adaptive criteria are proposed.The conversion between MD and SMD is very convenient and straightforward owing to their similarities.Two adaptive criteria,the centro-symmetry parameter criterion and the displacement criterion,are also proposed to automatically identify the regions requiring MD simulation,in which the displacement criterion is quite accurate and efficient.A multiple time step scheme and the above transition scheme are also adopted in proposed MD-SMD adaptive coupling method.Owing to similarities between SMD and MD or material point method(MPM),a concurrent multiscale method coupling MD and continuum-based MPM is proposed.An embedding region is adopted to eliminate the incompatibility between atoms and continuum.The proposed transition scheme is also adopted for seamless coupling.A multiple time step scheme is adopted for efficient concurrent coupling.Finally,parallel computing of the proposed MD-MPM coupling method is realized for better capability in handling large scale problems.