Quantum Theory of Advanced Materials

The work function of atomically uniform Ag films grown on Fe(100) is measured as a function of film thickness. It shows layer-resolved variations as a result of quantum confinement of the valence electrons. A first-principles calculation reproduces the observed variations except for very thin films (one and two monolayers), and the differences can be attributed, in part, to strain effects caused by the lattice mismatch between Ag and Fe. These results illustrate the close interaction between interface effects and surface properties.

We present a first-principles investigation of the lattice dynamics and thermodynamical properties of a complex hydride NaAlH(4), a promising material for hydrogen storage. The calculations are performed within the density-functional-theory framework and using a linear response theory. Calculations of the phonon spectrum, Born effective charges Z(*), and dielectric constants in high and low frequency limits are reported. The mode characters of the zone-center phonons, including the LO-TO splitting, are identified and compared to the experiment. The quasiharmonic approach is used to study thermal expansion as well as the mean square displacement of each atom as a function of temperature. A connection is established between the latter and the melting point. The inclusion of the zero-point motion leads to an expanded lattice compared to the static lattice, while the low frequency oscillations are found to play an important role in the melting and decomposition of NaAlH(4).

We present a first-principles investigation of the structural properties, electronic structure, and the chemical stability of the complex hydrides NaAlH(4) and Na(3)AlH(6). The calculations are performed within the density functional framework employing norm conserving pseudopotentials. The structural properties of both hydrides compare well with experimental data. A detailed study of the electronic structure and the charge-density redistribution reveal the features of an ionic covalent bonding between Al and H in the (AlH(4))(-) and (AlH(6))(-3) anionic complexes embedded in the matrix of Na(+) cations. The orbital hybridization and the characteristics of bonding orbitals within the complexes are identified. The calculated reaction energies of these complex hydrides are in good agreement with the experimentally determined values.

We have studied the pair-correlation function, the exchange-correlation hole, and the exchange-correlation energy density of the valence electrons in the Si atom using the Coulomb-coupling constant integration technique with the variational quantum Monte Carlo method. These quantities are compared to those derived from various approximate models within the Kohn-Sham density functional theory. We find that the charge density prefactor in the expression for the exchange-correlation hole dominates the errors found in the local spin density approximation (LSDA), that the generalized gradient approximation improves energy calculations by improving the LSDA at long ranges, and that the weighted spin density approximation, which uses the correct charge density prefactor, gives the lowest root mean square error for the exchange-correlation energy density.