1.12.3.1 Why Atomic-Scale Modeling?

First principle ab initio methods for self-consistent calculation of electron-density distribution around moving ions provide the most accurate modeling tech­niques to date. They take into account local chemical and magnetic effects and provide significant potential for predicting material properties. They are used with success in applications where the properties are limited to the nanoscale, for example, microelectronics, cataly­sis, nanoclusters, and so on (Chapter 1.08, Ab Initio Electronic Structure Calculations for Nuclear Materials). A typical scale for this is of the order of a few nm. However, this leaves a significant gap between ab initio methods and those required to model proper­ties of bulk materials arising from radiation damage. These involve phenomena acting over much longer scales, such as interactions between mobile and sessile defects, their thermally activated transport, their response to internal and external stress fields and gra­dients of chemical potential. Models for bulk properties are based on continuum treatments by elasticity, ther­mal conductivity, and rate theories where global defect properties such as formation, annihilation, transport, and interactions are already parameterized at contin­uum level. The only technique that currently bridges the gap in the scales between ab initio and the contin­uum is computer simulation of a large system of atoms, up to 106—108. Atoms move as in classical Newtonian dynamics due to effective forces between them calcu­lated from empirical interatomic potentials and respond to internal and external fields due to tempera­ture, stress, and local imperfections. Atomic-scale mod­eling has provided the results presented in this chapter. In the following section, we present a short description of typical models for simulation of dislocations and their interactions with defects formed by radiation.