Quantum Theory of Advanced Materials

Lithium imide (Li2NH) has been considered as a promising medium for hydrogen storage with the following reaction: LiNH(2)+LiH <-> Li(2)NH+H(2). All possible phases involved in the reaction need to be fully characterized in order to understand the right pathway connecting the two end compounds LiNH(2) and Li(2)NH and to further improve its reaction condition to meet the requirements of practical applications. We study from first-principles calculations the possible intermediate compounds Li(2-x)NH(1+x) between Li(2)NH and LiNH(2). Based on the energetics results, possible intermediate phases are identified for 0

Accurate multideterminant ground-state energies of circular quantum dots containing N <= 13 electrons as a function of interaction strength have been evaluated by the diffusion quantum Monte Carlo method. Two unique features are found for these confined two-dimensional systems: (1) as the electron density decreases, the quantum dots favor states with zero orbital angular momentum (L = 0); and (2) for some values of N, the ground state cannot be fully spin-polarized because of a symmetry constraint.

We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.

The charge-density-wave transition in TiSe2, which results in a commensurate (2 X 2 X 2) superlattice at temperatures below similar to 200 K, presumably involves softening of a zone-boundary phonon mode. For the first time, this phonon-softening behavior has been examined over a wide temperature range by synchroton x-ray thermal diffuse scattering.