My STSM Experience – Petr Tous

My STSM Experience – Petr Tous

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Over the summer of 2024, I had the incredible opportunity to participate in a short-term scientific mission, conducted at the University of Southampton under the leadership of prof. Graeme M. Day. The STSM lasted three months in total, with the goal of developing a CSP methodology for crystals of ionic liquids (ILs), or crystal organic salts, in other words. In general, salts represent a desirable target for material sciences as well as methods of CSP and as such should be represented in the studied materials in the BEST-CSP COST Action.

The city of Southampton has long and rich history as a port city, with memorials of various sea captains and crews scattered throughout, including the crew of the Titanic, since many of the ship’s crew originated in Southampton. And while the Highfield campus of the multi-faceted University of Southampton contains architecture that one thinks of when imagining “a British University,” many buildings have been modernized or built anew, making it worthy of a contemporary metropolitan city.

Prof. Day and his group , full of young and enthusiastic researchers, have contributed heavily to the field of CSP. Most recently, they have released their CSP code by the name of mol-CSPy for public use. The code is based on quasi-random structure generation with a subsequent force field-based optimization, where intramolecular interactions are described quantum-mechanically, while intermolecular interactions are described via a force field. My task was to utilize the code to perform CSP for ILs and subsequently develop a pipeline of reranking methods to reproduce results for ILs with a known crystal structure. This was used on ILs with either an unknown crystal structure or ILs that were not reported to form crystal forms at all. This arrangement had mutual benefits: I could learn to use a new piece of software directly from the people that helped develop it or have experience using it and they could in turn see how their code performs on other HPCs and with chemical compounds for which it was not necessarily designed. Additionally, I also performed calculations with a currently private, more advanced version of mol-CSPy with added functionality and could report bugs or problems right away.

Being able to consult with prof. Day or any of the members of his group undeniably brought me more insights into how modern CSP is performed and what have we learned about crystal packing over the years. Since CSP of ILs is the topic of my PhD thesis, all this acquired knowledge will be well utilized in my future work and career. As for the current results: We established that the current code is able to not only produce reasonable structures for co-crystals of ionic molecules but also reproduce already known crystal structures in the energy–density landscape. The experimental structures can be found in a small energy window at the bottom of the landscapes. However, they are not positioned as the most stable structures and thus, re-ranking is in order. Currently, we utilized a periodic DFT-based PBE-D3/PAW calculation with a medium-sized basis set for simple structure re-optimization, followed by a zero-point energy correction and so far, the re-ranking results have been promising in the sense that the experimental structures have reached a more favourable position in the overall ranking. We realize that a full-scale thermal free energy correction is going to be necessary, as ionic liquids exhibit polymorphic behaviour and thus the ranking needs to be precise. Currently, we are looking at several possibilities of how to perform such correction – a rigorous computation of phonon frequencies and a less expensive free energy calculation relying on basic laws of thermodynamics. We also repeated the steps thus taken for ILs that currently have no known experimental structure, providing us with promising candidates for their crystal structures. That being said, we plan to continue the work beyond the end of the STSM, including potential publications. Currently, I have also submitted a poster to the 8th Users’ Conference of IT4Innovations, to be presented at the beginning of November 2024.

It ended up being an exciting three months of my life. I would like to wholeheartedly thank prof. Day and his group members for all the invaluable experience I was able to obtain and for welcoming me so kindly into their ranks. I very much look forward to meeting again in the prospect future. I would also like to thank the organizers of the BEST-CSP COST Action for awarding me the grant so I could perform this STSM. And finally, I would like to thank the weather during my stay for not being too… well… British.

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