We present first-principle calculations on the vertical ionization potentials (IPs), electron affinities (EAs), and singlet excitation energies on an aromatic-molecule test set (benzene, thiophene, 1,2,5-thiadiazole, naphthalene, benzothiazole, and tetrathiafulvalene) within the GW and BetheSalpeter equation (BSE) formalisms. Our computational framework, which employs a real-space basis for ground-state and a transition-space basis for excited-state calculations, is well-suited for high-accuracy calculations on molecules, as we show by comparing against G0W0 calculations within a plane-wave-basis formalism. We then generalize our framework to test variants of the GW approximation that include a local-density approximation (LDA)-derived vertex function (ΓLDA) and quasiparticle-self-consistent (QS) iterations. We find that ΓLDA and quasiparticle self-consistency shift IPs and EAs by roughly the same magnitude, but with opposite sign for IPs and same sign for EAs. G0W0 and QSGW ΓLDA are more accurate for IPs, while G0W0ΓLDA and QSGW are best for EAs. For optical excitations, we find that perturbative GW -BSE underestimates the singlet excitation energy, while self-consistent GW -BSE results in good agreement with previous best-estimate values for both valence and Rydberg excitations. Finally, our work suggests that a hybrid approach, where G0W0 energies are used for occupied orbitals and G0W0ΓLDA for unoccupied orbitals, also yields optical excitation energies in good agreement with experiment but at a smaller computational cost.

We use a real-space implementation of pseudopotentials within the density-functional theory to investigate the structural and magnetic properties of cobalt clusters with up to 365 atoms. We find from structural optimization that hexagonal close-packed (hcp) and icosahedral clusters are lower in energy than body-centered cubic (bcc) and face-centered cubic (fcc) ones. We find the calculated magnetic moments generally decrease as a function of increasing cluster size. For clusters of several hundred atoms the bulk limit becomes apparent. However, the decrease is not monotonic. It depends on the details of the interior structure of the cluster and the corresponding surface geometry. By analyzing the detailed evolution of the local magnetic moment, we find the spin moment is bulk-like in the cluster interior and increases in the vicinity of the surface and can be correlated with coordination. The calculated behavior accounts for the observed variations in the measured moments.

Work function values measured at different surfaces of a metal are usually different. This raises an interesting question: What is the work function of a nano-size crystal, where differently oriented facets can be adjacent? Work functions of metallic nanocrystals are also of significant practical interest, especially in catalytic applications. Using real space pseudopotentials constructed within density functional theory, we compute the local work function of large aluminum and gold nanocrystals. We investigate how the local work function follows the change of the surface plane orientation around multifaceted nanocrystals, and we establish the importance of the orbital character near the Fermi level in determining work function differences between facets.

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We a employ real-space pseudopotential method to compute the structural energies of a prototypical system metal-semiconductor interface. Specifically, we examine a Pb(111) film overlaid on a Si(111) substrate as a function of the metal thickness. For each layer of Pb we fully relax the atomic coordinates and determine the lowest energy structure. Owing to the lattice mismatch between the Pb and Si crystal structures, we consider a large supercell containing up to 1,505 atoms for the largest system. Systems of this size remain challenging for most current computational approaches and require algorithms specifically designed for highly parallel computational platforms. We examine the structural properties of the interface with respect to the thickness of the metal overlayer, e.g., the corrugation of the profile of the Pb overlayer. The combined influence of the Si substrate and quantum confinement results in a rich profile for a transition between a thin overlayer (less than a few monolayers) where the corrugation is strong, and the bulk region, (more than a half dozen layers) where the overlaid Pb film is atomically flat. This work proves the feasibility of handling systems with such a level of complexity.