Increasing the QUANTUM ESPRESSO Capabilities II: Towards the TDDFT Simulation of Metallic Nanoparticles

This work regards the enabling of the Time-Dependent Density Functional Theory kernel (TurboTDDFT) of Quantum-ESPRESSO package on petascale systems. TurboTDDFT is a fundamental tool to investigate nanostructured materials and nanoclusters, whose optical properties are determined by their electronic excited states. Enabling of TurboTDDFT on petascale system will open up the possibility to compute optical properties for large systems relevant for technological applications. Plasmonic excitations in particular are important for a large range of applications from biological sensing, over energy conversion to subwavelength waveguides. The goal of the present project was the implementation of novel strategies for reducing the memory requirements and improving the weak scalability of the TurboTDDFT code, aiming at obtaining an important improvement of the code capabilities and to be able to study the plasmonic properties of metal nanoparticle (Ag, Au) and their dependence on the size of the system under test. The Scientific Challenge The optical properties of nanostructured materials and nanoclusters are determined by their electronically excited states. In particular, plasmonic excitations are particularly relevant for a large range of applications such as biological sensing, energy conversion and subwavelength waveguides [1-4]. In solid-state physics the electronic excitations are classified as single-particle excitations and plasmons. The distinction of plasmons and singleparticle excitations can be motivated from the homogeneous electron gas model. Plasmons in extended systems are usually well-described by classical electrodynamics (e.g. Drude-Lorentz model) [5]. For molecules and nanoparticles classical models are no longer applicable and a full quantum-mechanical treatment is necessary [6,7]. In this case, the optical spectrum does not show a single strong plasmon feature, but instead a spectrum with distinct peaks due to transitions between discrete energy levels, typical of a finite system. When increasing the size, the optical spectrum curve obtained from full quantum-chemical calculations is expected to approach the single plasmon feature of the one predicted by classical models. Density-functional theory (DFT) is considered the state of the art in the molecular modelling of materials at the atomic (nano) scale, allowing one to simulate models consisting of several hundred up to a few thousand atoms. The situation is not nearly as favourable for those spectroscopies, such as optical and UV absorption, which probe electronic excitations. Time-dependent (TD) DFT is emerging as alternative route to extend the scope of DFT to dynamical processes [8]. However, its numerical complexity has long been considered a restricting bottleneck, resulting in a substantial (one to two orders of magnitude) reduction of the size of systems that could be addressed. Thus, so far the standard realm of TDDFT was the study of small (tens to hundred atoms) molecules or clusters.