THEORY AND PRACTICE OF SHAPE SPECTROSCOPY OF LOCAL FCC STRUCTURES IN COMPUTER-SIMULATIONS OF NUCLEATION AND CRYSTALLIZATION

Starting from the previously developed probabilistic method for recogn ition of structures of small clusters of atoms undergoing thermal fluc tuations, we derive simple algorithms for the very detailed study of l ocal face-centered-cubic (FCC) arrangements of the atoms, directly in a configuration of atoms when it is being simulated. This includes (i) an algorithm for a local identification of FCC -like clusters, with t he discussion of its reliability, and (ii) an algorithm for the calcul ation of the relative orientational correlations between the clusters. As an illustration, the method is used for studying nucleation and gr owth of an FCC phase in a rapidly quenched melt of 4000 Lennard-Jones atoms. On the basis of this analysis, we propose two trial hypotheses about (i) the existence of ''slow'' and ''quick'' regimes during the n ucleation and growth and (ii) that in the intermediate stages of nucle ation and crystallization the FCC-like local order appears rather in t wo-, than in three-dimensional aggregates.