On the 'mature' wind force and a 'laggard' direct force in electromigration

Regarding the two different contributions to the driving force, the direct force and the wind force! the role of the electronic structure has been qui te different for the two. For the wind force increasingly sophisticated des criptions have been used, namely pseudopotential models, finite cluster mod els and, at the end, an ab initio korringa-Kohn-Rostoker (KKR) Green's func tion description. This will be illustrated by showing for which systems by now the wind force has been calculated. Almost all FCC and BCC metals are i ncluded, while both self-electromigration and impurity migration have been treated. Some new results will be presented as well, which simulate electro migration along a grain boundary and over a surface. The direct forcer on the other hand, has mainly been discussed in terms of the simple free electron, or jellium model. However, it: will be shown that we have arrived at a point, at which more sophisticated descriptions of th e electronic structure involved are becoming important. A recent analysis o f new experimental results leads to the conclusion that a migrating hydroge n atom effectively can have a direst valence smaller than unity depending o n the metal studied. By this it becomes challenging to perform calculations of the electronic structure of an interstitial, not only at its equilibriu m position, but also at positions lying along the jump path.