First principles optics: MgxTi1-xH2 continued, simple hydrides revisited and something else

The presentation will consist of two parts. In the first part we discus the optical properties of MgxTi(1-x)H2. We present dielectric functions constructed from both intra and inter band transitions, calculated in super cells simulation random alloys. We show that in super cells of 32 formula units already a significant changes in both inter and intra band contributions occur with respect to those of simple cells with the same Mg:Ti ratio. The inter band dielectric function is smoothened and the square of the intra band plasma frequency, which determines the intra band dielectric function, decrease by an order of magnitude. These two changes together offer an explanation for the nature of a black state. In the second part we discuss the dielectric functions of the simple hydrides of NaH, LiH and MgH2 calculated in the framework of GW/BSE theory. Previously we presented DFT/RPA dielectric functions which were shifted to fit the G0W0 band gap. Pure DFT, as usually, underestimated the band gap and the G0W0 corrected ones were slightly too large when compared to experimental values. We show here that at GW/BSE level, which includes exciton effects and uses a self consistent GW, both the band gap and the shape of the dielectric function is in agreement with the experimental results. We discuss the liberation modes of the octahedral AlH6 clusters in Na3AlH6. NMR experiments have suggested a free rotation mode at room temperature. We mapped the two dimensional space of rotational axes for a single cluster in the lattice of Na3AlH6 to obtain the axis with the lowest energy barrier. Finally we calculated the phonon modes in both the ground state and the transition state to determine, using classical transition state, the attempt frequency and predict the temperature above which an orientational anisotropy should be visible in NMR experiments