Relativistic quantum chemical calculations show that the uranium molecule U2 has a quadruple bond


Understanding the bonding, reactivity and electronic structure of actinides is lagging behind that of the rest of the periodic table. This can be partly explained by the challenges that one faces in experimental studies of such radioactive compounds and also by the need to properly account for relativistic effects in theoretical studies. A further challenge is the very complicated electronic structures encountered in actinide chemistry, as vividly illustrated by the naked diuranium molecule U2. Here we report a computational study of this emblematic molecule using state-of-the-art relativistic quantum chemical methods. Notably, the variational inclusion of spin–orbit interactions leads not only to a different electronic ground state, but also to a lower bond multiplicity compared with those in previous studies.

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Fig. 1: Frontier atomic valence orbital energies of uranium in non-relativistic, scalar relativistic and fully relativistic molecular orbital theories.
Fig. 2: CASSCF potential energy curves for the lowest electronic states of U2 around the equilibrium structure.

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All relevant data that are not included in this article and its Supplementary Information are available from the corresponding author upon reasonable request.


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We thank the Danish Center for Scientific Computing for ample computational resources. We thank T. Helgaker and E. Uggerud (University of Oslo) for help with Gorokhov et al.34. We thank C. K. Elmar for preparing the graphical abstract. S.K. acknowledges fruitful discussions with P. Pyykkö (University of Helsinki) on numerous occasions. S.K. thanks M. Reiher (ETH Zürich) for his continuous support. We dedicate this paper to the memory of B. Roos, an outstanding quantum chemist.

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S.K. ran all calculations reported in this paper, whereas the interpretation of the results and the preparation of the manuscript was a joint effort by all three authors. The generalized EBO (equation (2)) was developed by S.K. and T.S.

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Correspondence to Stefan Knecht.

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Knecht, S., Jensen, H.J.A. & Saue, T. Relativistic quantum chemical calculations show that the uranium molecule U2 has a quadruple bond. Nature Chem 11, 40–44 (2019).

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