One of the main sources of error associated with the calculation of defect formation energies using plane-wave density functional theory (DFT) is finite size error resulting from the use of relatively small simulation cells and periodic boundary conditions. Most widely used methods for correcting this error, such as that of Makov and Payne, assume that the dielectric response of the material is isotropic and can be described using a scalar dielectric constant $$\epsilon${}$. However, this is strictly only valid for cubic crystals, and cannot work in highly anisotropic cases. Here we introduce a variation of the technique of extrapolation based on the Madelung potential that allows the calculation of well-converged dilute limit defect formation energies in noncubic systems with highly anisotropic dielectric properties. As an example of the implementation of this technique we study a selection of defects in the ceramic oxide Li${}_{2}$TiO${}_{3}$ which is currently being considered as a lithium battery material and a breeder material for fusion reactors.

The rearrangement of a material's electron density during laser irradiation leads to modified nonthermal forces on the atoms that may lead to coherent atomic motions and structural phase transformation on very short time scales. We present ab initio molecular dynamics simulations of a martensitic solid-solid phase transformation in tungsten under conditions of strong electronic excitation. The transformation is ultrafast, taking just over a picosecond, and follows the tetragonal Bain path. To examine whether a solid-solid bcc-fcc phase transformation could occur during laser irradiation, we use two-temperature molecular dynamics (2T-MD) simulations with a specially developed potential dependent on the electronic temperature. Our simulations show that the occurrence of the solid-solid phase transformation is in competition with ultrafast nonthermally assisted melting with the strength of the electron-phonon coupling determining the lifetime of the new solid phase. In tungsten the melting transition is predicted to occur too rapidly for the fcc phase to be detectable during laser irradiation.