Solving the strong-correlation problem in materials

This article is a short introduction to the modern computational techniques used to tackle the many-body problem in materials. The aim is to present the basic ideas, using simple examples to illustrate strengths and weaknesses of each method. We will start from density-functional theory (DFT) and the Kohn–Sham construction—the standard computational tools for performing electronic structure calculations. Leaving the realm of rigorous density-functional theory, we will discuss the established practice of adopting the Kohn–Sham Hamiltonian as approximate model. After recalling the triumphs of the Kohn–Sham description, we will stress the fundamental reasons of its failure for strongly-correlated compounds, and discuss the strategies adopted to overcome the problem. The article will then focus on the most effective method so far, the DFT+DMFT technique and its extensions. Achievements, open issues and possible future developments will be reviewed. The key differences between dynamical (DFT+DMFT) and static (DFT+U) mean-field methods will be elucidated. In the conclusion, we will assess the apparent dichotomy between first-principles and model-based techniques, emphasizing the common ground that in fact they share.

Applications of large deviation theory in geophysical fluid dynamics and climate science

The climate is a complex, chaotic system with many degrees of freedom. Attaining a deeper level of understanding of climate dynamics is an urgent scientific challenge, given the evolving climate crisis. In statistical physics, many-particle systems are studied using Large Deviation Theory (LDT). A great potential exists for applying LDT to problems in geophysical fluid dynamics and climate science. In particular, LDT allows for understanding the properties of persistent deviations of climatic fields from long-term averages and for associating them to low-frequency, large-scale patterns. Additionally, LDT can be used in conjunction with rare event algorithms to explore rarely visited regions of the phase space. These applications are of key importance to improve our understanding of high-impact weather and climate events. Furthermore, LDT provides tools for evaluating the probability of noise-induced transitions between metastable climate states. This is, in turn, essential for understanding the global stability properties of the system. The goal of this review is manifold. First, we provide an introduction to LDT. We then present the existing literature. Finally, we propose possible lines of future investigations. We hope that this paper will prepare the ground for studies applying LDT to solve problems encountered in climate science and geophysical fluid dynamics.

The Italian contribution to the construction of the linac for the European spallation source

The European spallation source (ESS) uses a linear accelerator (linac) to deliver the high intensity proton beam to the target station for producing intense beams of neutrons. At the exit of the linac, the proton beam will have 2 GeV energy and 62.5 mA current. The construction of an accelerator with the contribution of different laboratories is not a new concept but so far the laboratories were controlled by the same government (e.g. in USA and Japan) or they delivered components for an intergovernmental institution like CERN. The European Spallation Source is a research facility that gathers 40 active in-kind (IK) contributors from 13 States, even outside the European Union, so its construction is not only a technical and scientific challenge, but also an economic, political and social experiment. The case of the Italian contribution is interesting because of the structure of Italian industrial ecosystem, mostly based on small and medium-sized enterprises (SME), which may be unsuitable for the case of a research infrastructure which construction requires a high level of R&D investments. Conversely, the well-known flexibility of SME to adapt to the requirements have balanced the weakness and the results are satisfactory. Following the overview of the Linac design, the paper will focus on the key issues of the Italian contribution, the state of the project (73% completion up to now) along with the point of view of the ESS management and the lesson learnt; the major outcomes for the economy and society will complete the discussion.