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Breaking the curse of dimension in heavy-element chemistry

Periodic Reporting for period 1 - PCCDX (Breaking the curse of dimension in heavy-element chemistry)

Reporting period: 2016-07-01 to 2018-06-30

An essential element of actinide-based research is the prediction of the stability and properties of actinide-containing compounds. Since their acute toxicity, radioactivity, and instability complicate experimental studies on actinide compounds, theoretical approaches have to be used to determine their properties and reactivity. Unfortunately, conventional computational models are difficult, primarily because the computational resources required grow unfavourably with the size of the system, an effect known as the curse of dimension. Thus, innovative new approaches must be developed that break the curse of dimension. One such approach describes molecules as a collection of noninteracting electron pairs, called geminals. Conventional geminal-based methods developed so far are, however, inappropriate for actinide chemistry. The main objective of this project is, thus, to extend geminal-based models to be applicable to actinide chemistry. To accomplish this task, we need to include (i) computationally efficient ways to account for relativistic effects, (ii) correlations between electrons beyond electron-pairing effects, (iii) the modeling of electronically excited states, and (iv) the description of unpaired electrons. The developed models should be robust, computationally cheap, reliable, and black-box-like, requiring minimal user-software interplay. These technical advantages compared to standard approaches will facilitate theoretical modeling of actinide-containing materials out of reach of present-day quantum chemistry methods and will be of crucial importance for a fundamental understanding of actinide chemistry. For instance, the extended geminal models will provide the essential insights that are needed to guide the synthesis of new actinide compounds that can be used to separate Uranium and Plutonium from the other components in the soup of nuclear waste. Furthermore, the scope of applications of the proposed new quantum mechanical model can be easily extended to other areas of chemistry and material physics like lanthanide and transition-metal chemistry, biochemical reactions, and semiconductor physics.
Our numerical studies demonstrate that all new quantum mechanical methods that have been developed in this project provide an improved, atomistic, and quantitative computational model to describe actinide chemistry. In general, our methods outperform all conventional quantum mechanical approaches used in computational chemistry that are suitable for actinide-containing compounds, while simultaneously the computational scaling could be significantly reduced. To conclude, the proposed geminal-based models are a robust, computationally inexpensive, and user-friendly alternative to standard, more complicated computational approaches.
In this project, we (i) implemented computationally efficient ways to account for relativistic effects, (ii) derived and implemented new models to account for correlations between electrons beyond electron-pairing effects, (iii) developed alternative models to target electronically excited states, and (iv) extended the original electron-pair model to also describe quantum states with unpaired electrons. All our developments are implemented in our open-source quantum chemistry software package called PIERNIK which will be released in summer/fall 2019. All methods developed in this project were tested and benchmarked for various challenging molecules that contain elements across the periodic table. Specifically, we studied small actinide molecules that represent building blocks of larger actinide compounds encountered, for instance, in nuclear waste reprocessing. Our numerical studies suggest that our developments (i) to (iv) result in reliable and robust quantum mechanical models that can be considered as an inexpensive and accurate alternative to conventional methods in computational chemistry. For instance, the quantum chemistry methods proposed in this project facilitate a balanced description of the correlated motion of electrons in both ground and excited states. Furthermore, the proposed geminal-based models allow us to accurately capture so-called doubly-excited electronic states, that is quantum states that feature a transfer of two electrons at the same time. The latter is particularly difficult to describe using conventional approaches at low computational cost. All results were presented at various international scientific conferences of relevance to theoretical and computational chemistry and were published in peer-reviewed journals. Furthermore, we performed several outreach activities to promote science, especially quantum chemistry, to the general public (science camp of the Polish Children’s fund, science festival in Kutno, various press releases).
We developed and implemented novel quantum chemistry methods that are applicable to ground and excited states of heavy-element containing compounds that contain both paired and unpaired electrons. All proposed models were implemented in an open-source quantum chemistry software suite that will be available for the general public free of charge. The proposed geminal-based methods represent robust, reliable, inexpensive, and user-friendly computational models that can be applied to many challenging areas of chemistry, including heavy-element chemistry (transition-metal, lanthanide, and actinide chemistry) as well as materials physics. Thus, the proposed models can be applied to, for instance, guide the synthesis of new (actinide) compounds or to accurately predict ground and excited state properties of large, realistic materials.
Overview on all pCCD extensions developed on this project