Freezing of Small SeF6 Clusters: Simulations, Nucleation Statistics When Events Are Few, and Effects of Laplace Pressure

Simulations were carried out on 138-molecule clusters freezing isothermally at 130, 120, and 80 K. At 120 K, the nucleation rate was the same as in our prior simulations performed adiabatically but the final product was different. During the nanosecond period of the runs, clusters transforming adiabatically had frozen to bcc crystals while warming from 120 K to about 130 K. On the other hand, isothermal clusters at 120 and 130 K changed to monoclinic clusters after passing through the bcc phase. Clusters cooled to 80 K froze to a variety of structures. The number of molecules whose Voronoi polyhedra qualified them as being in bcc embryos grew in size erratically, and in most runs it was difficult to use the Voronoi information by itself to identify a well-characterized nucleation time. Therefore, a more discriminating criterion for the onset of nucleation was devised. The 138-molecule clusters proved to be too small to yield definitive profiles of the several order parameters characterizing the change fro...