From the atomistic and electronic structure point of view, it is legitimate to distinguish between electrically conducting materials on one hand and insulating or semiconducting materials on the other. Indeed, insulating materials exhibit specific behaviors, especially for the point defects. Due to the existence of a gap in the electronic density of states, the point defects may be charged. There is recent evidence that the properties of the point defects, especially their kinetic properties, such as the migration energy, depend a lot on their charge state. The charge of a given point defect depends on the position of the Fermi level within the band gap: a low lying Fermi level (close to the valence band) favors positively charged defects, whereas a Fermi level close to the conduction band favors negatively charged defects. The positions of the Fermi level corresponding to transitions between charge states are called charge transition levels (CTL). The correct determination of these CTL allows the correct prediction of the charge states of the defects, as piloted by Fermi level position, that is, the doping conditions for the material. Standard DFT methods fail to reproduce accurately these CTL, and the research of more accu­rate methods is presently a very active field in the electronic structure community, with major implica­tions for microelectronic research as well as for nuclear materials, especially in view of the aforementioned variation of point defect kinetic properties with their charge state. All these charge aspects of point defects are completely out of range for empirical potentials.

In the last two sections, we exemplify the research on insulating materials by summarizing the available results for two important insulating nuclear materi­als: silicon carbide and uranium dioxide.

Silicon carbide is an important candidate material for fusion and fission applications. Even if it arguably a less crucial material than UO2, we start with this material as its electronic structure is simpler. UO2 is obviously the basic model material for the nuclear fuel of usual reactors.