MOLECULAR-DYNAMICS STUDY OF A CRYSTALLINE CLUSTER UNDERGOING A 2ND-ORDER TRANSITION - AN INCHOATE MODEL OF ACETYLENE

To extend our molecular dynamics studies of nucleation in molecular cl usters undergoing phase changes, we selected the acetylene system whos e 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 struct ure of the low-temperature orthorhombic phase, a new interaction poten tial was developed which successfully yielded the correct molecular pa cking at low temperatures as well, as the cubic structure observed at high temperatures. The results obtained in heating and cooling runs fo r the model system were unlike any seen in our prior simulations. Ther e was no nucleation barrier inhibiting the passage from the cubic to t he orthorhombic phase on cooling, nor were there discernible volume or enthalpy changes in the transformation. In clusters of several hundre d molecules the apparently second-order or continuous transition was s pread over a much wider range of temperatures than had been seen in fi rst-order transitions in previous simulations. Although the results pr ovide an instructive example of an unusual type of transformation, the y do not appear to correspond to the transition observed experimentall y for pure, bulk acetylene.