The activation–relaxation technique: an efficient algorithm for sampling energy landscapes

Activated processes, i.e., rare events requiring thermal fluctuations many times larger than the average thermal energy, play a central role in controlling the relaxation and diffusion mechanisms of disordered materials such as amorphous and glassy solids, polymers and bio-molecules. As the time scales involved are much longer than those associated with thermal vibrations, these processes cannot be studied efficiently with standard real-space methods such as molecular dynamics (MD). They can be investigated much more efficiently by working in the potential energy space. Instead of defining moves in terms of atomic displacements, the activation–relaxation technique (ART) follows paths directly in the energy landscape, from local minima to adjacent saddle points, giving full freedom for the system to create events of any complexity. In this paper, we review the technique in detail and present some recent applications to amorphous semiconductors and glasses.