# Tag: conference

## VSC-users day 2019

It is becoming an interesting yearly occurrence: the VSC user day. During this 5th edition, HPC users of the various Flemish universities gather together at the Belgian Royal Academy of Science (KVAB) to present their state-of-the-art work using the Flemish Tier-1 and Tier-2 supercomputers. This is done during a poster-presentation session. This year, I presented my work with regard to vibrational spectra in solids and periodic systems. In contrast to molecules, vibrational spectra in solids are rarely investigated at the quantum mechanical level due to their high cost. I show that imaginary modes are not necessarily a result of structural instabilities, and I present a method for identifying the vibrational spectrum of a defect.

In addition, international speakers discuss recent (r)evolutions in High Performance Computing, and during workshops, the participants are introduced in new topics such as GPU-computing, parallelization, and the VSC Cloud and data platform. The possibilities of GPU were presented by Ehsan, of the VSC, showing extreme speedups of 10x to 100x, strongly depending on the application, the graphics card. It is interesting to see that simple CUDA prama’s can be used to obtain such effects…maybe I should have a go at them for the Hirshfeld and phonon parts of my HIVE code…if they can deal with quadruple precision, and very large arrays. During the presentation of Joost Vandevondele (ETH Zürich) we learned what the future holds with regard to next generation HPC machines. As increasing speed becomes harder and harder to obtain, people are again looking into dedicated hardware systems, a situation akin to the founding days of HPC. Whether this is a situation we should applaud remains to be seen, as it means that we are moving back to codes written for specific machines. This decrease in portability will probably be alleviated by high level scripting languages (such as python), which at the same time result in a significant loss of the initial gain. (Think of the framework approach to modern programming which leads to trivial applications requiring HPC resources to start.)

In addition, this year the HPC-team of the TIER-1 machine is present for a panel discussion, presenting the future of the infrastructure. The machine nearly doubled in size which is great news. Let us hope that in addition for financing hardware, there is also a significant budget considered for a serious extension of a dedicated HPC support team. Running a Tier-1 machine is not something one does as a side-project, but which requires a constant vigilance of a dedicated team to deal with software updates, resulting compatibility issues, conflicting scripts and just hardware and software running haywire because they can.

With this hope, I look toward the future. A future where computational research is steadily are every quickly is becoming common place in the fabric om academic endeavors.

## MRS seminar: Topological Insulators

Bart Sorée receives a commemorative frame of the event. Foto courtesy of Rajesh Ramaneti.

Today I have the pleasure of chairing the last symposium of the year of the MRS chapter at UHasselt. During this invited lecture, Bart Sorée (Professor at UAntwerp and KULeuven, and alumnus of my own Alma Mater) will introduce us into the topic of topological insulators.

This topic became unexpectedly a hot topic as it is part of the 2016 Nobel Prize in Physics, awarded last Saturday.

This year’s Nobel prize in physics went to: David J. Thouless (1/2), F. Duncan M. Haldane (1/4) and J. Michael Kosterlitz (1/4) who received it

##### “for theoretical discoveries of topological phase transitions and topological phases of matter.”

On the Nobel Prize website you can find this document which gives some background on this work and explains what it is. Beware that the explanation is rather technical and at an abstract level. They start with introducing the concept of an order parameter. You may have heard of this in the context of dynamical systems (as I did) or in the context of phase transitions. In the latter context, order parameters are generally zero in one phase, and non-zero in the other. In overly simplified terms, one could say an order parameter is a kind of hidden variable (not to be mistaken for a hidden variable in QM) which becomes visible upon symmetry breaking. An example to explain this concept.

##### Example: Magnetization of a ferromagnet.

In a ferromagnetic material, the atoms have what is called a spin (imagine it as a small magnetic needle pointing in a specific direction, or a small arrow). At high temperature these spins point randomly in all possible directions, leading to a net zero magnetization (the sum of all the small arrows just lets you run in circles going nowhere). This magnetization is the order parameter. At the high temperature, as there is no preferred direction, the system is invariant under rotation and translations (i.e. if you shift it a bit or you rotate it, or both you will not see a difference) When the temperature is lower, you will cross what is called a critical temperature. Below this temperature all spins will start to align themselves parallel, giving rise to a non-zero magnetization (if all arrows point in the same direction, their sum is a long arrow in that direction). At this point, the system has lost the rotational invariance (because all spins point in  direction, you will know when someone rotated the system) and the symmetry is said to have broken.

Within the context of phase transitions, order parameters are often temperature dependent. In case of topological materials this is not the case. A topological material has a topological order, which means both phases are present at absolute zero (or the temperature you will never reach in any experiment no matter how hard you try) or maybe better without the presence of temperature (this is more the realm of computational materials science, calculations at 0 Kelvin actually mean without temperature as a parameter). So the order parameter in a topological material will not be temperature dependent.

#### Topological insulators

To complicate things, topological insulators are materials which have a topological order which is not as the one defined above 😯 —yup why would we make it easy 🙄 . It gets even worse, a topological insulator is conducting.

OK, before you run away or loose what is remaining of your sanity. A topological insulator is an insulating material which has surface states which are conducting. In this it is not that different from many other “normal” insulators. What makes it different, is that these surface states are, what is called, symmetry protected. What does this mean?

In a topological insulator with 2 conducting surface states, one will be linked to spin up and one will be linked to spin down (remember the ferromagnetism story of before, now the small arrows belong to the separate electrons and exist only in 2 types: pointing up=spin up, and pointing down=spin down). Each of these surface states will be populated with electrons. One state with electrons having spin up, the other with electrons having spin down. Next, you need to know that these states also have a real-space path let the electrons run around the edge of material. Imagine them as one-way streets for the electrons. Due to symmetry the two states are mirror images of one-another. As such, if electrons in the up-spin state more left, then the ones in the down-spin state move right. We are almost there, no worries there is a clue. Now, where in a normal insulator with surface states the electrons can scatter (bounce and make a U-turn) this is not possible in a topological insulator. But there are roads in two directions you say? Yes, but these are restricted. And up-spin electron cannot be in the down-spin lane and vice versa. As a result, a current going in such a surface state will show extremely little scattering, as it would need to change the spin of the electron as well as it’s spatial motion. This is why it is called symmetry protected.

If there are more states, things get more complicated. But for everyone’s sanity, we will leave it at this.  😎

## tUL Life Sciences Research Day 2016

Yesterday was the tUL Life Sciences Research Day 2016. A conference event build around finding collaboration possibilities between the University of Hasselt in Belgium and the University of Maastricht (The Netherlands)…after all tUL is the “transnational University Limburg” which brings two universities together that are only separated some 26 km, but you have to cross a national border.

Although Life sciences itself is not my personal niche, I went to look for opportunities, as nano-particles which are used for drug delivery often consist of metals or oxides. These materials on the other hand are my niche. I used my current work on MOFs as a means to show what is possible from the ab-initio point of view, and presented this as a poster.

Poster presented at the tUL Life Sciences Research Day, depicting my work on the unfunctionalized and the functionalized MIL-47(V) MOF.

## Colloquium on Porous Frameworks: Day 2

On Monday, we had the second day of our colloquium on Porous Frameworks, containing no less than 4 full sessions, covering all types of frameworks. We started the day with the invited presentation of Prof. Dirk De Vos of the KU Leuven, who discussed the breathing behavior in Zr and Ti containing MOFs, including the work on the COK-69 in which I was involved myself. In the MOFs presented, the breathing behavior was shown to originate from the folding of the linkers, in contrast to breathing due to the hinging motion of the chains in MIL-47/53 MOFs.

After the transition metals, things were stepped up even further by Dr. Stefania Tanase who talked about the use of lanthanide ions in MOFs. These lanthanides give rise to coordinated water molecules which appear to be crucial to their luminescence. Prof. Donglin Jiang, of JAIST in Japan, changed the subject to the realm of COFs, consisting of 2D porous sheets which, through Van Der Waals interactions form 3D structures (similar to graphite). The tunability of these materials would make them well suited for photoconductors and photoenergy conversion (i.e. solar cells).

With Prof. Rochus Schmid of the University of Bochum we delved into the nitty-gritty details of developing Force-Fields for MOFs. He noted that such force-fields can provide good first approximations for structure determination of new MOFs, and if structure related terms are missing in the force-field these will pop up as missing phonon-frequencies.

Prof. Monique Van der Veen showed us how non-polar guest molecules can make a MOF polar, while Agnes Szecsenyi bravely tackled the activity in Iron based MIL-53 MOFs from the DFT point of view. The row of 3 TU Delft contributions was closed by the invited presentation of Prof. Jorge Gascon who provided an overview of the work in his group and discussed how the active sites in MOFs can be improved through cooperative effects.

Prof. Jaroslaw Handzlik provided the last invited contribution, with a comparative theoretical study of Cr-adsorption on various silicate based materials (from amorphous silicate to zeolites). The final session was then closed by the presentations of Dr. Katrine Svane (Bath University) who discussed the effect of defects in UiO-66 MOFs in further detail and Marcus Rose presenting his findings on hyper-crosslinked Polymers, a type of COFs with an amorphous structure and a wide distribution in different pore sizes.

This brought us to a happy end of a successful colloquium, which was celebrated with a drink in the city center of Groningen. Tuesday we traveled back home, such that Wednesday Sylvia could start at the third part of the conference-holiday roller coaster by leaving for Saltzburg.

## Colloquium on Porous Frameworks: Day 1

Today the CMD26 conference started in Groningen, and with its kick-off also our own 2-day colloquium on porous frameworks (aka MOFs, COFs and Zeolites) was launched. During the two sessions of the day, the focus mainly went out to the Zeolites, with Prof. Emiel Hensen of the Technical university of Eindhoven introducing us to the subject and discussing how new zeolites could be designed in a more rational way. He showed us how the template used during synthesis plays a crucial role in the final growth and structure. Dr. Nakato explained how alkali-metal nanoclusters can undergo insulator to metal transitions when incorporated in zeolites (it is due to the competition between electron-electron repulsion and electron-phonon coupling), while Dr. De Wijs informed us on how Al T-sites need to be ordered and assigned in zeolites to allow for the prediction of NMR parameters.

After the coffee break Dr. Palcic, from the Rudjer Boskovic Institute in Croatia, taught us about the role of heteroatoms in zeolites. She told us that even though more than 2 million theoretical structures exist, only 231 have officially been recognized as having been synthesized, so there is a lot more work to be done. She also showed that to get stable zeolites with pores larger than 7-8 Angstrom one needs to have 3 and 4-membered rings in the structure, since these lead to more rigid configurations. Unfortunately these rings are themselves less stable, and need to be stabilized by different atoms at the T-sites.

Dr. Vandichel, still blushing from his tight traveling scheme, changed the subject from zeolites to MOFs, in providing new understanding in the role of defects in MOFs on their catalytic performance. Dr. Liu changed the subject even further with the introduction of COFs and showing us how Hydrogen atoms migrate through these materials. Using the wisdom of Bruce Lee :

You must be shapeless, formless, like water. When you pour water in a cup, it becomes the cup. When you pour water in a bottle, it becomes the bottle. When you pour water in a teapot, it becomes the teapot.

he clarified how water behaves inside these porous materials. Our first colloquium day was closed by Ir. Rohling, who took us back to the zeolite scene (although he was comparing the zeolites to enzymes). He discussed how reactivity in zeolites can be tweaked by the confinement of the reacting agents, and how this can be used for molecule identification. More importantly he showed how multiple active site collaborate, making chemical reactions much easier than one would expect from single active site models.

After all was said and done, it was time to relax a little during the conference welcome reception. And now time to prepare for tomorrow, day 2 of our colloquium on porous frameworks.

## Holiday-Conference roller coaster

Visit to Stockholm. The knight at the Medeltidsmuseet (top left), brown bear in Skansen (top right), visiting the Royal palace (bottom left) and local entertainment in the old city center (bottom right).

Summertime is a time of rest for most people. For our little academic family, last summer was a bit of a roller coaster; alternating holidays with hard work which had been postponed too much. The last vestige of my start of a new chapter (moving the remaining stuff from the apartment to our house) was finally bested. Now the conference roller coaster has started with Sylvia’s plenary lecture on conceptual spaces in Stockholm.

As neither of us ever visited Sweden before, we decided to turn it into a semi-family-holiday as well. Our 4-year-old son enjoyed his first ever plane flight (he wasn’t really convinced something impressive was going on). And while Sylvia was of to the conference, the two of us went to explore Stockholm: Finding the knight in the Medeltidsmuseet (at the left in the back of this beautiful museum 🙂 ) and searching for the king and queen at their palace (they weren’t there 🙁 ). Or visiting one of the oldest open-air musea; Skansen (similar to Bokrijk in Belgium) where we saw old professions at work (making cheese for example) and native Scandinavian farm and wild animals (from peacocks to brown bears).

Next weekend starts the next episode of the conference roller-coaster with me hosting a 2-day colloquium on porous frameworks together with Bartek Szyja and Ionut Tranca at the CMD-26 conference in Groningen. We have a nicely packed colloquium with about 20 presentations (8 invited and 12 contributed) covering the whole realm of porous materials from zeolites to COFs and MOFs. The program of the colloquium can be downloaded below:

## Annual Meeting of the Belgian Physical Society 2016

Wednesday May 18th was a good day for our little family. Since my girlfriend an I both are physicists by training, we attended the annual meeting of the Belgian Physical Society in Ghent, together. What made this event even more special was the fact that both of us had an oral presentation at the same conference, which never happened before. 🙂

Sylvia talked about an example of indeterminism in Newtonian mechanics, and showed how the indeterminism can be clarified by using non-standard analysis. The example considers the Norton Dome, a hill with a specifically designed shape ( $y(x)=-2/3(1-(1-3/2|x|)^{2/3})^{3/2}$ ). When considering a point mass, experiencing only gravitational force, there are two solutions for the equation of motion: (1) the mass is there, and remains there forever (r(t)=0) and (2) the mass was rolling uphill with a non-zero speed which becomes exactly zero at the top, and continues over the top ( $r(t)=\frac{1}{144} (t-T)^4$ with T the time the top is reached). Here, r refers to the arc length as measured along the dome (0 at the top). In addition, there also exists a family of solutions taking the first solution at t<T, while taking the second solution at t>T. (As the first and second derivatives of these latter solutions are continuous, Newton will not complain.) This leads to indeterminism in a Newtonian system; for instance, you start with a mass on the top of the hill, and at a random point in time it starts to roll off without the presence of an external something putting it into motion. Using infinitesimals, Sylvia shows that the probability for the mass to start rolling off the dome immediately is infinitesimally close to one.

My own talk was on the use of computational materials science as a means for understanding and explaining experimental observations. I presented results on the pressure-induced breathing of the MIL-47(V) MOF, showing how the experimentally observed S-shape of the transition-pressure-curve can be explained by the spin interactions of the unpaired vanadium-d electrons: it turns out that regions with only ferromagnetic chains compress already at 85 MPa, while the addition of higher and higher percentages of anti-ferromagnetic chains increases the pressure at which the pores collapse, up to 125 MPa for the regions containing 100% anti-ferromagnetic chains. As a second topic, I showed how the electronic band structure of the linker-functionalized UiO-66(Zr) MOF changes. When one or two -OH or -SH groups are added to the benzene ring of the linker, part of the valence band is split off and moves into the band gap. In semiconductors, this would be called a gap state; however, in this case, since every linker in the material contributes

Top left: I am presenting computational results on MOFs. Top Right: Sylvia presents the Norton Dome. Bottom: Group picture at the central garden in “Het Pand”. (Photos: courtesy of Sylvia Wenmackers (TL), Philippe Smet (TR), and Michael Tytgat (B) )

a single electron state to this gap state, it practically becomes the valence band top. As a consequence, the color of such functionalized MOF’s changes from white to yellow and orange. As a third topic, I discussed the COK-69(Ti) MOF. In this MOF the electrons in the titaniumoxide clusters are strongly correlated, just as for pure titaniumoxide. Because such systems are poorly described with standard DFT, we used the DFT+U approach, which allowed us to discern between Ti3+ and Ti4+ ions. The latter was practically done by partitioning the electron density using the Hirshfeld-I scheme.

Following these plenary presentations, four young scientists competed for the young speaker award presenting their PhD research. Two presentations (1),(2) focused on vortices in superconductors, a third one discussed the use of plasmons in graphene nanoribbons to enhance telecommunication while the fourth talk introduced us into the world of string theory.

In the afternoon, there were six parallel session, of which I mainly attended the Condensed Matter and Nanostructure Physics-session (since I had my own talk there) and the Biological, Medical, Statistical and Mathematical Physics-session rooting for Sylvia. During the Condensed matter session I was mainly fascinated by the presentation of Prof. Sara Bals, on coloring atoms in 3 dimensions. She showed how, using energy-dispersive X-ray (EDX) mapping it is possible to create a 3D atomic lattice of nano-materials and clusters. This is a more direct approach than the usual X-ray diffraction (XRD) approach for identifying a crystal structure. Unfortunately, I am afraid this technique may not be well suited for the MOFs I’m working on, since they contain mainly light elements and not heavy metals(although it may be interesting to try once the technique is optimized further). It is, however, definitely a technique to remember for future projects, to suggest to experimental collaborators.

## Call for Abstracts: Condensed Matter Science in Porous Frameworks: On Zeolites, Metal- and Covalent-Organic Frameworks

Together with Ionut Tranca (TU Eindhoven, The Netherlands) and Bartłomiej Szyja (Wrocław University of Technology, Poland) I am organizing a colloquium “Condensed Matter Science in Porous Frameworks: On Zeolites, Metal- and Covalent-Organic Frameworks” which will take place during the 26th biannual Conference & Exhibition CMD26 – Condensed Matter in Groningen (September 4th – 9th, 2016). During our colloquium, we hope to bring together experimental and theoretical researchers working in the field of porous frameworks, providing them the opportunity to present and discuss their latest work and discoveries.

Zeolites, Metal-Organic Frameworks, and Covalent-Organic Frameworks are an interesting class of hybrid materials. They are situated at the boundary of research fields, with properties akin to both molecules and solids. In addition, their porosity puts them at the boundary between surfaces and bulk materials, while their modular nature provides a wealthy playground for materials design.

We invite you to submit your abstract for oral or poster contributions to our colloquium. Poster contributions participate in a Best Poster Prize competition.

#### The extended deadline for abstract submission is May 14th, 2016.

CMD26 – Condensed Matter in Groningen is an international conference, organized by the Condensed Matter Division of the European Physical Society, covering all aspects of condensed matter physics, including soft condensed matter, biophysics, materials science, quantum physics and quantum simulators, low temperature physics, quantum fluids, strongly correlated materials, semiconductor physics, magnetism, surface and interface physics, electronic, optical and structural properties of materials. The scientific programme will consist of a series of plenary and semi-plenary talks and Mini-colloquia. Within each Mini-colloquium, there will be invited lectures, oral contributions and posters.

Feel free to distribute this call for abstracts and our flyer and we hope to see you in Groningen!

## SBDD XXI

Today was the first day of the three-day long diamond conference at the university of Hasselt. And although this sounds as-if it is a mere small-scale local conference, it is actually one of the two main international conferences in the field. The Surface and Bulk Defects in Diamond (SBDD) workshop grew in twenty years from a small event with only a few dozen participants to the current event with over 200 participants. As such, it is the place to be, for one as me, who is dipping into a new field of materials.

One thing that already became quite clear today, is the fact that there are many opportunities in this field for the computational materials scientist, as the large majority of the researchers are experimentalists. Of the >120 posters presented, I have only discovered about 5 theoretical ones. Having had very nice chats with their presenters I already learned a lot of what I will have to keep in mind when studying diamond. But so far, I have not come across any issues that are impossible to resolve, which is good news :-).

## Virtual Winterschool 2016: Computational Solid State Physics & Chemistry

In just an hour, I’ll be presenting my talk at the virtual winterschool 2016. In an attempt to tempt fate as much as possible I will try to give/run real-time examples on our HPC in Gent, however at this moment no nodes are available yet to do so. Let’s keep our fingers crossed and see if it all works out.

### Abstract

Modern materials research has evolved to the point where it is now common practice to manipulate materials at nanometer scale or even at the atomic scale (e.g. Intel’s skylake architecture with 14nm features, atomic layer deposition and surface structure manipulations with an STM-tip). At these scales, quantum mechanical effects become ever more relevant, making their prediction important for the field of materials science.

In this session, we will discuss how advanced quantum mechanical calculations can be performed for solids and indicate some differences with standard quantum chemical approaches. We will touch upon the relevant concepts for performing such calculations (plane-wave basis-sets, pseudo-potentials, periodic boundary conditions,…) and show how the basic calculations are performed with the VASP-code. You will familiarize yourself with the required input files and we will discuss several of the most important output-files and the data they contain.

At the end of this session you should be able to set up a single-point calculation, a structure optimization, a density of states and band structure calculation.