In a number of areas of application, the behavior of systems depends sensitively on properties that pertain to the atomistic scale, i.e., the angstrom and femtosecond scales. Specifically, atomistic details are important where material response is highly non-linear, e.g., at atomistically sharp crack tips and other stress concentrators, near atomic-scale defects such as vacancies and dislocations and at other atomistically sharp features such as free surfaces, grain boundaries and material interfaces (Gill, 2010). However, often the properties and behaviors of interest are macroscopic and take place on the scale of centimeters to meters, and are characterized by slow evolution on the scale of minutes to years. A case in point is degradation in nuclear reactor materials, which results from a combination of heat, irradiation, stress and corrosion exposure for extended periods of time (20 or more years, cf., e.g., Marquis et al., 2009 and references therein). This vast disparity of length and time scales poses extraordinary challenges in theoretical and computational material science.
Molecular Dynamics (MD) and Monte Carlo (MC) methods are powerful techniques to study deformation and diffusion mechanisms in systems of particles, but they are limited to relatively small material samples and to time windows of microseconds at best (e.g., Sutton et al., 1992 and Wang et al., 1992). Considerable effort has been devoted to accelerating MD and MC methods and notable successes have been recorded in that direction (cf., e.g., Voter et al., 2002). However, no computationally tractable atomistically based models appear to be as yet available to study slow phenomena, over time scales of the order of minutes to years, while maintaining a strictly atomistic description of the materia