In a series of recent experiments, Alivisatos and coworkers demonstrated that the pressure-induced transition from the four-coordinate wurtzite structure to the six-coordinate rocksalt structure in CdSe nanocrystals is strongly influenced by crystal size. On a molecular time scale, the transition is a rare event and the resulting long time scales present a challenge for computer simulations [1]. Using this example, I will discuss how the transition path sampling methodology provides a framework to treat systems with widely disparate time scales and, in particular, to study the mechanism and the kinetics of first order phase transformations close to experimental conditions [2]. From our transition path sampling simulation we obtain the preferred transformation pathway and determine activation enthalpies and volumes, which permit to make contact to experimental results.   


[1] M. Grünwald, E. Rabani, and C. Dellago,  "Mechanisms of the wurtzite to rocksalt transformation in CdSe nanocrystals", Physical Review Letters 96, 255701 (2006).

[2] M. Grünwald, P. L. Geissler, and C. Dellago,  "An efficient transition path sampling algorithm for nanoparticles under pressure", Journal of Chemical Physics 127, 154718 (2007).


 

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1. Transition states in pressure-induced structural transformations of nanocrystals