Optical absorption spectra of the three lowest-energy isomers of Ag n ͑n =10,12-20͒ are investigated from first principles within the time-dependent local-density approximation ͑TDLDA͒. The computed spectra are found to be generally in good agreement with the available experimental data. The analyses of the spectra indicate that the d electrons of Ag n clusters in this size range have a significant ͑70% -80%͒ contribution to low-energy optical excitations. We show that most of the peak positions and the relative intensities in the TDLDA spectra of these subnanometer sized clusters can be explained remarkably well within the classical Mie-Gans theory, using the dielectric function of bulk Ag and taking into account the shapes of the isomers.

To reduce fuel cell cost, durable and inexpensive electrode catalysts need to be developed to replace precious metal materials, particularly for the electrocatalytic oxygen reduction at cathodes. In this study, we explored the structure and the energetics of Fe-xN (x ) 2,4) incorporated into carbon nanotubes and graphene using density functional theory to show that these structures are more stable than iron atoms on nanotubes and that pyridinic structures of Fe-4N are more favorable than pyrrolic structures. EXAFS spectra simulated from the optimized structures show good agreement with results of measurements obtained on arrays of aligned nanotubes doped with iron and nitrogen, which have demonstrated activity toward oxygen-reduction reactions.

Energies from the GW approximation and the Bethe-Salpeter equation (BSE) are benchmarked against the excitation energies of transition-metal (Cu, Zn, Ag, and Cd) single atoms and monoxide anions. We demonstrate that best estimates of GW quasiparticle energies at the complete basis set limit should be obtained via extrapolation or closure relations, while numerically converged GW-BSE eigenvalues can be obtained on a finite basis set. Calculations using real-space wave functions and pseudopotentials are shown to give best-estimate GW energies that agree (up to the extrapolation error) with calculations using all-electron Gaussian basis sets. We benchmark the effects of a vertex approximation (Γ) and the mean-field starting point in GW and the BSE, performing computations using a real-space, transition-space basis and scalar-relativistic pseudopotentials. While no variant of GW improves on perturbative GW at predicting ionization energies, GWΓ-BSE computations give excellent agreement with experimental absorption spectra as long as off-diagonal self-energy terms are included. We also present GW quasiparticle energies for the CuO, ZnO, AgO, and CdO anions, in comparison to available anion photoelectron spectra.

Optical absorption spectra for the computed ground state structures of copper clusters (Cun, n = 2 − 20) are investigated from first principles using time-dependent density functional theory in the adiabatic local density approximation (TDLDA). The results are compared with available experimental data, existing calculations, and with results from our previous computations on silver and gold clusters. The main effects of d electrons on the absorption spectra, quenching the oscillator strengths and getting directly involved in low-energy excitations, increase in going from Agn to Aun to Cun due to the increase in the hybridization of the occupied, yet shallow, d orbitals and the partially occupied s orbitals. We predict that while Cu nanoparticles of spherical or moderately ellipsoidal shape do not exhibit Mie (surface plasmon) resonances unlike the case for Ag and Au, extremely prolate or oblate Cu nanoparticles with eccentricities near unity should give rise to Mie resonances in the lower end of the visible range and in the infra-red. This tunable resonance predicted by the classical Mie-Gans theory is reproduced with remarkable accuracy by our TDLDA computations on hypothetical Cu clusters in the form of zigzag chains with as few as 6 to 20 atoms.