Energy landscapes of some model glass formers - art. no. 024205

The potential energy surfaces of several model atomic glass formers have be en studied using eigenvector-following techniques. Barrier distributions, c ooperativity indices, path lengths, and vibrational densities of states (VD OS) are presented based upon data sets containing more than 250 000 pathway s in total. We find that rearrangements can usefully be separated into "non diffusive" processes, which do not change the nearest-neighbor contacts and "diffusive" processes, which do. We suggest a criterion to separate these classes: nondiffusive processes are those in which no atoms move more than a threshold distance. Energy barriers for the two classes of rearrangement differ much more in the "strong" system (Stillinger-Weber silicon) than in the "fragile" Lennard-Jones systems. Our results indicate that the system i s not trapped in a single local minimum below the glass transition temperat ure, because there are numerous "nondiffusive" rearrangements with low barr iers still accessible. Disconnectivity graphs for low-energy regions of the potential energy surface illustrate how the crystal is rapidly located onc e a critical nucleus is present. Finally, the calculated VDOS show a pronou nced excess over the Debye density of states in the low-frequency region. T ransition state searches following the eigenvectors corresponding to these soft modes converge to low-lying transition states, including some that sep arate nearly degenerate minima. This result provides support for the hypoth esis that two-level systems and the boson peak are related.