Category Archives: Reports

Report: CECAM/Psi-k Workshop Bremen on Reliable and quantitative prediction of defect properties in Ga-based semiconductors

CECAM Report

Program GASC

Organizers: Thomas Frauenheim (Bremen), Peter Déak (Bremen), Klaus Irmscher (Berlin), Susanne Siebentritt (Luxembourg),  Joel B. Varley (Livermore, California)

Venue: University of Bremen, Bremen Center for Computational Materials Science (BCCMS), Germany, 8th until 12th of October 2018

Sponsors: University of Bremen (BCCMS), Psi-k, DFG

Defect engineering in micro/optoelectronics and in photovoltaics has immensely profited from electronic structure calculations. In the past two decades, local and semi-local approximations of density functional theory were the workhorses of theoretical studies but, by now, it has become clear that they do not allow a sufficiently accurate and reliable prediction of defect properties in wide band gap materials. While ab initio energy methods for calculating the total energy are struggling with the system sizes necessary for defect modeling, semi-empirical methods using various corrections or hybrid functionals are being applied for the purpose. While the theoretical background of these methods and their relation to each other is by now more or less understood, the transferability of the semi-empirical parameters and the overall predictive power is still unclear. Progress requires further systematic testing and comparison of the various methods, as well as validation against experiments. For that, accurate measurement data on defects are needed on a set of materials, which are structurally or compositionally related. Gallium based semiconductors, like GaN, Ga2O3 and CuInxGa1-xSe(S)2 chalcogenides (CIGS) offer a good possibility for testing theory and are interesting also experimentally due to their versatile applications.

While there are still open defect-related questions in the much studied blue LED-material GaN, very few defects could be positively identified as yet in CIGS solar cell materials, while the research on the potential power semiconductor and UV transparent electrode Ga2O3 has barely started. Following the successful workshops on Gallium Oxide and Related Materials, held in Kyoto (Japan) in 2015, and in Parma (Italy) in 2017, as well as several workshops on chalcogenide photovoltaic materials, e.g., Symposium V at the E-MRS Spring meeting 2016, the CECAM-workshop in Bremen 2018 will focus on bringing together experimentalist interested in gallium oxide and Ga-based chalocgenides with theorists who are active in the field. A friendly and stimulating environment will facilitate discussions, adding impetus to both the development of practically applicable theoretical methods and to progress in the defect engineering of these materials.

Workshop Report:”Modern Approaches to Coupling Scales in Materials Simulation”

Our workshop was held from the 2nd to the 4th of July at the Hotel Jäger von Fall which sits on a peninsula on the Sylvensteinsee, an artificial lake in the Bavarian foothills of the Alps near the Austrian border. This is a very remote location chosen deliberately to allow participants to concentrate fully on the scientific program. Transportation from and to the nearest train station was accomplished with a shuttle-bus courtesy of the Technical University of Munich (TUM), driven by some of the participating TUM members. For the most part the weather was very inviting, inspiring a small number of participants to take a refreshing swim during the breaks, it turns out that the water was very clear but also very, very cold.

The two evenings of the workshop where filled with a poster-session on Monday, held outdoors due to the lovely weather, and a conference dinner, again outdoors, on Tuesday.

The event was supported by Psi-k, the German science foundation (DFG), the international graduate school of science and engineering (IGSSE), as well as the Technical University of Munich.

Workshop Report: “Theoretical methods in molecular spintronics” (TMSpin)

The Workshop “Theoretical methods in molecular spintronics (TMSpin) was held at the Materials Physics Center of the University of the Basque Country in Donostia-San Sebastian from the 17th to the 20th of September 2018. This workshop welcomed 31 invited speakers and several postgraduate students presenting posters. The event was co-sponsored by Psi-k and the Donostia International Physics Centre (DIPC- http://dipc.ehu.es/).

Molecular spintronics is the study of spin-related phenomena in molecules and atoms and their possible applications for the next generation of data storage and processing devices as well as for the implementation of quantum computers. Electronic structure theory has played a prominent role in molecular spintronics. The comparison of theory and experiments has demonstrated the importance of first-principles calculations, which go beyond model representations of molecular devices as simple “quantum dots” or effective spin Hamiltonians. Nonetheless, standard electronic structure methods based on Density Functional Theory often fail in describing molecular spintronic systems even at a qualitative level. This is because most magnetic phenomena are manifestations of correlation effects, which become extreme at the single molecule scale and which are not captured by standard implementations and approximations of DFT. The goal of TMSpin was to address the question:

“What electronic structure theory to use for molecular spintronics?”

The workshops gathered theoretical physicists and quantum chemists with different areas of expertise. On the one hand there were those researchers that have provided important contributions to the advancement of molecular spintronics since its inception. They were asked to give an overview about the field and moreover to highlight the open questions that to date cannot yet be addressed by theory. On the other hand, the workshop gathered some of the leading researchers in theory and code development, who presented the most recent fundamental and numerical advancements for a number of methods. The organizers promoted an intense discussion to understand whether such methods can be already employed in molecular spintronics. Continue reading Workshop Report: “Theoretical methods in molecular spintronics” (TMSpin)

Report of the E-CAM workshop “Improving the accuracy of ab-initio methods for materials”

Title: Improving the accuracy of ab-initio predictions for materials
Location: CECAM-FR-MOSER
Webpage with list of participants, schedule and slides of presentations: http://www.cecam.org/workshop-0-1643.html
Dates: September 17, 2018 to September 20, 2018
Organizers: Dario Alfè, Michele Casula, David CeperleyCarlo Pierleoni

State of the art
Improving the accuracy of ab-initio methods for materials means to devise a global strategy which integrates several approaches to provide a robust, controlled and reasonably fast methodology to predict properties of materials from first principle. Kohn-Sham DFT is the present workhorse in the field but its phenomenological character, induced by the approximations in the exchange-correlation functional, limit its transferability and reliability.
A change of paradigm is required to bring the ab-initio methods to a predictive level. The accuracy of XC functional in DFT should be assessed against more fundamental theories and not, as it is often done today, against experiments. This is because the comparison with experiments is often indirect and could be misleading. The emerging more fundamental method for materials is Quantum Monte Carlo because of: 1) its favourable scaling with system size with respect to other Quantum Chemistry methods; 2) its variational character which defines an accuracy scale and allows to progressively improve the results. However QMC being much more demanding in terms of computer resources, and intricate than DFT, a combined approach is still desirable where QMC is used to benchmark DFT approximations for specific systems before performing the production study by DFT.
A different aspect of accuracy is related to size effects: often relevant phenomena occurs at length and time scales beyond the one approachable by first-principle methods. In these cases effective force fields methods can be employed. Machine Learning methods can be used to extract those force fields from training sets provided by ab-initio calculations. Presently DFT-based training sets are used. Improving their accuracy will improve the ultimate accuracy at all scales.
This change of paradigm requires building a community of people with different expertises working in an integrated fashion. This has been the main aim of the workshop.

Continue reading Report of the E-CAM workshop “Improving the accuracy of ab-initio methods for materials”

Report: MSSC2018 – Ab initio Modelling in Solid State Chemistry

The Department of Chemistry and the Thomas Young Centre at Imperial College London and the Computational Materials Science Group of the Science and Technology Facilities Council (STFC), in collaboration with the Theoretical Chemistry Group of the University of Torino, organised the 2018 MSSC Summer School on the “ab initio modelling of crystalline and defective solids with the CRYSTAL code”.

CRYSTAL is a general-purpose program for the study of periodic solids. It uses a local basis set comprised of Gaussian type functions and can be used to perform calculations at the Hartree-Fock, density functional or global and range-separated hybrid functionals (e.g. B3LYP, HSE06), double hybrid levels of theory. Analytical first derivatives with respect to the nuclear coordinates and cell parameters and analytical derivatives, up to fourth order, with respect to an applied electric field (CPHF/CPKS) are available.

The school provided an overview of the underlying theory and fundamental issues affecting use of the code, with particular emphasis on practical issues in obtaining reliable data efficiently using modern computer hardware.  The capabilities of CRYSTAL was illustrated with hands-on tutorials organized in the afternoon sessions.

All information about the school can be found on this website:
http://www.imperial.ac.uk/mssc2018/

Read the full workshop report here: MSSC2018_Psi-k_report

 

Report: MSSC2017 – Ab initio Modelling in Solid State Chemistry

The Department of Chemistry and the Thomas Young Centre at Imperial College London and the Computational Materials Science Group of the Science and Technology Facilities Council (STFC), in collaboration with the Theoretical Chemistry Group of the University of Torino, organised the 2017 MSSC Summer School on the “ab initio modelling of crystalline and defective solids with the CRYSTAL code”.

The school provided an overview of the underlying theory and fundamental issues affecting use of the CRYSTAL code, with particular emphasis on practical issues in obtaining reliable data efficiently using modern computer hardware.

The capabilities of CRYSTAL was illustrated with hands-on tutorials organized in the afternoon sessions.

All information about the school can be found on this website:
http://www.imperial.ac.uk/mssc2017/

Read the full workshop report here: MSSC2017_Psi-k_report

Modern Approaches to Coupling Scales in Materials Simulation

Hotel Jäger von Fall, Lenggries, Bavaria, Germany
Organizers: Harald Oberhofer, Johannes Margraf
Webpage: http://macsims.ch.tum.de

Multi-scale simulation approaches rely on a hierarchy of increasingly accurate and highly resolved methods to capture the different time- and length-scales relevant to a process of
interest. Traditionally, this might involve coupling classical molecular dynamics with electronic structure calculations (QM/MM), or embedding a quantum mechanical system in a point charge
or continuum environment. In this context, the models comprising the individual layers of the multi-scale hierarchy are often unrelated. For instance, the empirical potential and DFT method in a QM/MM simulation are independently defined at the beginning of the simulation. Enormous advances in electronic structure algorithms and hardware now allow first principles calculations to be carried out on a truly massive scale. This leads to a novel perspective of multi-scale models: electronic structure data can be generated with high enough quality and quantity to allow the application of coarse graining and machine learning techniques. Instead of defining
separate physical models at different scales, the electronic structure method directly informs the next layer of the multi-scale hierarchy. The goal of this workshop was to bridge the gap between
traditional, layered multi-scale techniques and the more direct coarse graining and machine learning approaches to the simulation of extended systems, thereby bringing together researchers working on QM/MM or other embedding techniques with those who apply coarse graining and interpolation to electronic structure data in different contexts (e.g. potential energy surfaces, electronic properties, charge transport, rate constants in catalysis) and with different methods (neural networks, Gaussian process regression, kernel ridge regression, splining, etc).

Read the full report here.

CECAM/Marvel/Psi-k School on Path Integral Quantum Mechanics: From the Basics to the Latest Developments

Organisers: Michele Ceriotti, Tom Markland, Jeremy Richardson and Mariana Rossi

Dates: 25 -29 June, 2018

We convened a School on Path Integral Quantum Mechanics at the CECAM headquarters in Lausanne, Switzerland. The school gathered together 17 speakers (11 invited and 6 contributed) and 46 participants affiliated with 15 different countries. We
received a total of 85 applications to attend the school and unfortunately could not accept more participants due to space constraints in the lecture room. This amount of applications, only two years after we had the last school on the same topic, underlines
the growth of the community performing research on the theory and practice of Path Integral (PI) techniques for the atomic-scale modelling of the quantum behavior of materials and molecules.

As in the last school, we explicitly asked the speakers to prepare pedagogic talks aimed at introducing the participants to the methods and simulation techniques to treat imaginary and real time path integrals, for both adiabatic and non-adiabatic dynamics.
Invited and contributed speakers were encouraged to give lectures that explained the methods in great detail, so that the students could benefit the most from the school, even if this was their first contact with path integral methods.

Read the full report here.

Workshop on Interfacing Machine Learning and Experimental Methods for Surface Structures (IMPRESS)

July, 11-13th 2018, Graz University of Technology, Petersgasse 16,  8010 Graz, Austria

Introduction

In the second week of July, the workshop Interfacing Machine  Learning and Experimental Methods for Surface Structures  (IMPRESS) was held at the TU Graz. The advent of machine learning  methods has drastically changed the way structure determination is  performed, since it facilitates the rational design of (new)  experiments and the analysis of large amounts of data. The target of  the workshop was to bring experimentalists and theorists together,  so that both can learn and benefit from each other’s expertise.  About 50 scientists from Asia, America, and Europe followed the  call, making the workshop, which was sponsored by CECAM and the  Psi-k, a great success.

Continue reading Workshop on Interfacing Machine Learning and Experimental Methods for Surface Structures (IMPRESS)

Theoretical Spectroscopy Lectures

October 8, 2018 to October 12, 2018
Location: CECAM-HQ-EPFL, Lausanne.

The aim of the school was to give a deep introduction on the theoretical and practical aspects of the electronic excitations, which are probed by experimental techniques such as optical absorption, EELS and photo-emission (direct or inverse). From the theory point of view, excitations and excited state properties are out of the reach of density-functional theory (DFT), which is a ground-state theory. In the last thirty years, other ab-initio theories and frameworks, which are able to describe electronic excitations and spectroscopy, have become more and more used: time-dependent density-functional theory (TDDFT) and many-body perturbation theory (MBPT) or Green’s function theory (GW approximation and Bethe-Salpeter equation BSE). In fact, computational solutions and codes have been developed in order to implement these theories and to provide tools to calculate excited state properties. The present school focused on these points, covering theoretical, practical, and also numerical aspects of TDDFT and MBPT, non-linear reponse and real-time spectroscopies. Finally, a large part of the school was devoted to the codes implementing such theories (ABINIT, 2Light, Lumen, DP, EXC).

Continue reading Theoretical Spectroscopy Lectures