Modeling the Photo-Absorption Properties of Noble Metal Nanoclusters: A Challenge for Density-Functional Theory

Abstract

Modeling the emergence of the plasmon resonance in noble metal nanoclusters is still a challenge to overcome for theoretical chemistry. The systems are indeed too small to neglect quantum-size effects but too large to be easily addressed with quantum mechanics. We test here a robust answer to this still open question: the simplified variant to time-dependent density-functional theory (TDDFT). Applied to extended systems, this electronic structure-based method succeeds in computing a sufficient number of excitations to cover the emergence of plasmon-like states. By employing it under a semilocal exchange-correlation approximation such as PBE, we show that the most intense absorption band, which could be wrongly assigned to the plasmon band, has a strong interband character. We suspect the too low energy gap between (n–1)d and ns valence orbitals as the origin of the d-contamination of the excitations. We demonstrate however that a global or range-separated hybrid exchange-correlation approximation such as PBE0 or RSX-PBE0 is a robust answer to the problem. We notice that both approximations are not able to solve at the same time the energy positioning and intensity of the plasmon band, PBE0 being more accurate for energy positioning and RSX-PBE0 for intensity. All in all, we warn the user that a random choice of the exchange-correlation approximation opens the door to getting the correct answer for the wrong reason.

Optical Properties of AgAu Alloy Clusters: Effect of Chemical Configuration along a Rearrangement Pathway

“…As statistically converged DPS simulations are rather computer-intensive, an efficient semiempirical potential was needed to treat systems with as many degrees of freedom as a 55-atom cluster, and in the present work, the many-body potential based on secondmoment approximation (SMA) to tight-binding theory, parametrized for silver and gold by Rossi and coworkers [51], was employed to mimic the structural dependence of the electronic structure of Ag-Au nanoalloys. This approach is not expected to be quantitative against methods that explicitly account for electronic structure, such as DFT, or even density-functional-based tight-binding, although the difficulty of correctly describing noble metal clusters with DFT is also documented [52][53][54].…”