The computational study of chemical reactions in complex, wet environments is critical for applications in many fields. It is often essential to study chemical reactions in the presence of applied electrochemical potentials, taking into account the non-trivial electrostatic screening coming from the solvent and the electrolytes. As a consequence, the electrostatic potential has to be found by solving the generalized Poisson and the Poisson-Boltzmann equations for neutral and ionic solutions, respectively. In the present work, solvers for both problems have been developed. A preconditioned conjugate gradient method has been implemented for the solution of the generalized Poisson equation and the linear regime of the Poisson-Boltzmann, allowing to solve iteratively the minimization problem with some ten iterations of the ordinary Poisson equation solver. In addition, a self-consistent procedure enables us to solve the non-linear Poisson-Boltzmann problem. Both solvers exhibit very high accuracy and parallel efficiency and allow for the treatment of periodic, free, and slab boundary conditions. The solver has been integrated into the BigDFT and Quantum-ESPRESSO electronic-structure packages and will be released as an independent program, suitable for integration in other codes.

The BigDFT project was started in 2005 with the aim of testing the advantages of using a Daubechies wavelet basis set for Kohn–Sham (KS) density functional theory (DFT) with pseudopotentials. This project led to the creation of the BigDFT code, which employs a computational approach with optimal features of flexibility, performance, and precision of the results. In particular, the employed formalism has enabled the implementation of an algorithm able to tackle DFT calculations of large systems, up to many thousands of atoms, with a computational effort that scales linearly with the number of atoms. In this work, we recall some of the features that have been made possible by the peculiar properties of Daubechies wavelets. In particular, we focus our attention on the usage of DFT for large-scale systems. We show how the localized description of the KS problem, emerging from the features of the basis set, is helpful in providing a simplified description of large-scale electronic structure calculations. We provide some examples on how such a simplified description can be employed, and we consider, among the case-studies, the SARS-CoV-2 main protease.

In this review paper, several new approaches about the 3C-SiC growth are been presented. In fact, despite the long research activity on 3C-SiC, no devices with good electrical characteristics have been obtained due to the high defect density and high level of stress. To overcome these problems, two different approaches have been used in the last years. From one side, several compliance substrates have been used to try to reduce both the defects and stress, while from another side, the first bulk growth has been performed to try to improve the quality of this material with respect to the heteroepitaxial one. From all these studies, a new understanding of the material defects has been obtained, as well as regarding all the interactions between defects and several growth parameters. This new knowledge will be the basis to solve the main issue of the 3C-SiC growth and reach the goal to obtain a material with low defects and low stress that would allow for realizing devices with extremely interesting characteristics.

We investigated the complex dielectric function of bromide perovskite single crystals (MAPbBr₃, FAPbBr₃ and CsPbBr₃) by spectroscopic ellipsometry from 1 to 5 eV in the 183–423 K temperature range and under a dry nitrogen environment. The temperature dependence of all critical points provided a benchmark for the interband electronic transition energies in the three bromide perovskites for a wide spectral range. We found that the number of transitions in CsPbBr₃ depends on the crystallographic phase, with the orthorhombic lattice showing an extra transition with respect to the tetragonal and the cubic phases. Through density functional theory calculations, we identified the changes in the electronic structure that accompany the tetragonal-to-orthorhombic transition, leading to splitting of the complex dielectric function along the a and b crystallographic directions. We thereon used a similar experimental approach to study the robustness of bromide perovskites under humid air. We found that FAPbBr₃ and CsPbBr₃ remain unchanged up to 383 K, whereas MAPbBr₃ is irreversibly modified at 363 K. The degradation of MAPbBr₃ impacts its optical transitions and introduces a pregap absorption tail. Together with this, a dramatic damage of the surface is observed with the formation of reverse truncated pyramids accompanied by local worsening of the carrier extraction barrier in the remaining flat sample surface.