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Multiscale modeling of strain rate effects in molecular systems at finite temperature

Pan Xiao (Institute of Mechanics, CAS), Yilong Bai (LNM, Institute of Mechanics, CAS)

From Atomistics to Reality: Spanning Scales in Simulations and Experiments Symposium A

Tue 2:40 - 4:00

CIT 165

Since the establishment of quasicontinuum (QC) method proposed by Tamdor et al. in 1996, multiscale simulation has attracted considerable attention over the past years. Although the early QC version was limited to quasi-static problems at zero degree, since then, multiscale modeling of the dynamics of molecular systems at finite temperature appears to be a new challenge. Yet it is a troublesome issue, because of the huge gap between the time scales involved in molecular systems (about 10^-13 s, characterized by the atomic thermal oscillations) and the dynamic deformations in practices (like 10^-6 s in typical dynamic loadings). In this talk, we present some results and understandings on a new multiscale method of temperature and strain rate effects in molecular systems, in order to promote the development of multiscale modeling. In order to consider the temperature effect on quasi-static processes in large molecular systems, a multiscale algorithm named as molecule/cluster statistical thermodynamics (MST/CST) is developed, which is based on the minimization of Helmholtz free energy and the combined representation of molecules and clusters. The method has been applied to several molecular systems, such as ZnO nanowires, nanoindentation, etc. All these simulations show much higher efficiency than MD simulations, and the results of ZnO nanowires demonstrate very good agreement with experimental measurements. For the strain rate effects, the MD simulations show that the strain rate effect on deformation is closely related to the strong coupling of the evolving potential landscapes and the slow molecular oscillations. This microscopic mechanism, which may govern the practical rate effect, is examined in details in this talk. Based on this understanding, a combined MST/MD scheme is developed. It seems that the method is able to properly and efficiently simulate both temperature and strain rate effects simultaneously in molecular systems.