Assessing the Quality of QM/MM Approaches to Describe Vacuo-to-water Solvatochromic Shifts

The performance of different Quantum Mechanics/Molecular Mechanics embedding models to compute vacuo-to-water solvatochromic shifts are investigated. In particular, both non-polarizable and polarizable approaches are analyzed and computed results as compared to reference experimental data. We show that none of the approaches outperforms the others and that errors strongly depend on the nature of the molecular transition. Thus, we prove that the best choice of embedding model highly depends on the molecular system, and that the use a specific approach as a black-box can lead to significant errors and sometimes totally wrong predictions.Comment: 12 pages, 6 figure

Assessing the quality of QM/MM approaches to describe vacuo-to-water solvatochromic shifts

The performance of different quantum mechanics/molecular mechanics embedding models to compute vacuo-to-water solvatochromic shifts is investigated. In particular, both nonpolarizable and polarizable approaches are analyzed and computed results are compared to reference experimental data. We show that none of the approaches outperform the others and that errors strongly depend on the nature of the molecular transition to be described. Thus, we prove that the best choice of embedding model highly depends on the molecular system and that the use of a specific approach as a black box can lead to significant errors and, sometimes, totally wrong predictions. Published under an exclusive license by AIP Publishing

Struttura fine degli atomi idrogenoidi

Le correzioni ai livelli energetici dovuti alla struttura fine degli atomi idrogenoidi leggeri vengono calcolate utilizzando un approccio perturbativo non relativistico. All'operatore hamiltoniano imperturbato vengono affiancati tre termini correttivi per rendere conto delle piccole perturbazioni causate da effetti relativistici quali l'interazione spin-orbita, la variazione della massa relativistica dell’elettrone e la sua non localizzabilità, dovuta a zitterbewegung. La scala delle correzioni apportate dalla struttura fine, rispetto alle energia di Bohr, sono all'incirca dell'ordine di (Zα)^2, dove Z è il numero atomico e α la costante di struttura fine. Inoltre, la struttura fine riesce a rimuovere solo parzialmente la degenerazione dei livelli energetici di Bohr

QM/Classical Modeling of Surface Enhanced Raman Scattering Based on Atomistic Electromagnetic Models

We present quantum mechanics (QM)/frequency dependent fluctuating charge (QM/ωFQ) and fluctuating dipoles (QM/ωFQFμ) multiscale approaches to model surface-enhanced Raman scattering spectra of molecular systems adsorbed on plasmonic nanostructures. The methods are based on a QM/classical partitioning of the system, where the plasmonic substrate is treated by means of the atomistic electromagnetic models ωFQ and ωFQFμ, which are able to describe in a unique fashion and at the same level of accuracy the plasmonic properties of noble metal nanostructures and graphene-based materials. Such methods are based on classical physics, i.e. Drude conduction theory, classical electrodynamics, and atomistic polarizability to account for interband transitions, by also including an ad-hoc phenomenological correction to describe quantum tunneling. QM/ωFQ and QM/ωFQFμ are thus applied to selected test cases, for which computed results are compared with available experiments, showing the robustness and reliability of both approaches

Graphene Plasmonics: a Novel Fully Atomistic Approach for Realistic Structures

We demonstrate that the plasmonic properties of realistic graphene and graphene-based materials can effectively and accurately be modeled by a novel, fully atomistic, yet classical, approach, named $\omega$FQ. Such model is able to reproduce all plasmonic features of these materials, and their dependence on shape, dimension and fundamental physical parameters (Fermi energy, relaxation time and two-dimensional electron density). Remarkably, $\omega$FQ is able to accurately reproduce experimental data for realistic structures of hundreds of nanometers ($\sim$ 370.000 atoms), which cannot be afforded by any \emph{ab-initio} method. Also, the atomistic nature of $\omega$FQ permits the investigation of complex shapes, which can hardly be dealt with by exploiting widespread continuum approaches.Comment: 20 pages, 4 figure