Nucleation in Liquids and Amorphous Materials

One of the primary goals for the understanding of crystallization of liquids and amorphous materials is to gain a microscopic picture of the nucleation processes. MTD, which accelerates the occurrence of rare events, appears to be well suited for this purpose. 

Lennard-Jones systems are ideal starting models for nucleation studies. Since they have been extensively studied by a number of different approaches, they also serve as an excellent testing ground for MTD. In Sect.3.2.2.1, we present the first study of nucleation in Lennard-Jones Argon using MTD. 

In the subsequent subsections, we review recent results about MTD simulations of nucleation employing more sophisticated classical potentials (ice and NaCl) and ab initio methods (PCMs). 

Trudu el al. [32] performed both transition path sampling (TPS) [33] and MTD simulations of the Lennard-Jones model of Argon at pressure P = 0.25 kbar and temperatures ranging from 0.7 Tm to 0.8 Tm- The system size of 6912 atoms was considered to be sufficiently large to avoid significant periodic boundary effects (the expected nucleus size is Nc 200). 

Then the authors performed a commitment probability analysis (CPA) [33], which consists in assessing the probability for the system to go to the liquid or the crystalline state while changing randomly the velocities. This analysis enabled them to identify the transition state ensemble, i.e. the set of configurations from which the system evolves to the two phases with equal probability. This ensemble turned out to contain crystalline nuclei with a broad size distribution and far from spherical shapes. By averaging over the distribution, a critical size of Nc = 240 34 atoms was obtained. 

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