Molecular Dynamics Study of a Crystalline Cluster Undergoing a Second-Order Transition. An Inchoate Model of Acetylene

Abstract

To extend our molecular dynamics studies of nucleation in molecular clusters undergoing phase changes, we selected the acetylene system whose clusters had already been examined experimentally in supersonic flow. Because molecular dynamics simulations based on the several existing model potential functions were incapable of accounting for the structure of the low-temperature orthorhombic phase, a new interaction potential was developed which successfully yielded the correct molecular packing at low temperatures as well as the cubic structure observed at high temperatures. The results obtained in heating and cooling runs for the model system were unlike any seen in our prior simulations. There was no nucleation barrier inhibiting the passage from the cubic to the orthorhombic phase on cooling, nor were there discernible volume or enthalpy changes in the transformation. In clusters of several hundred molecules the apparently second-order or continuous transition was spread over a much wider range of temperatures than had been seen in first-order transitions in previous simulations. Although the results provide an instructive example of an unusual type of transformation, they do not appear to correspond to the transition observed experimentally for pure, bulk acetylene.