NONERGODICITY AND 2 SUBPHASES IN THE COEXISTENCE REGION IN ISOMERIZATION DYNAMICS OF AR-7-LIKE MOLECULES

It is well-known that a single cluster like Ar-7 undergoes ''melting'' from solidlike to liquidlike states as the energy is increased, the t ransition of which is not as sudden as the ordinary phase transition t hough and has a somewhat broad energy range in which solid and liquid coexist. We study a very anomalous dynamics of the coexistence region in the structural isomerization. It is explicitly shown that the time- series of the structural changes both in the purely solidlike and liqu idlike phases are stationary, while the coexistence region is found to generate a strongly nonstationary dynamics. The calculated distributi on of the residing times for the cluster to stay in one of the possibl e structures exhibits a nonexponential form having a large hole around the zero lifetime in the coexistence region. Motivated by these stran ge behaviors, we have calculated the phase-space volumes that are assi gned to the individual potential basins, and verified directly that wh ile the pure liquid region is of ergodic nature, the dynamics in the c oexistence region is indeed strongly nonergodic. The steep rises of th e Lindemann index and the maximum Liapunov exponent in the coexistence region, which were reported before by other authors, are found to be ascribed to the statistical nature rather than the dynamical propertie s as opposed to the picture suggested by the physical;meaning of the i ndices. It also turns out that the energy range for the coexistence re gion should be taken wider than considered before and thus extends bey ond the ''melting point'' that is defined usually on the basis of the Lindemann index. Therefore it is appropriate to divide the coexistence region into two subphases. A ''temperature'' in a microcanonical ense mble is defined so as to characterize the distribution of phase-space volume on a given energy plane. Based on this distribution, we describ e a statistical reason why the onset energy of the melting is much hig her than those of the transition states. (C) 1996 American Institute o f Physics.