Potential energy surfaces for the uranium hydriding reaction

We have computed the potential energy surfaces for the low-lying electronic states of uranium hydrides, $UHn$ $(n=1–3),$ which are important in the uranium hydriding reactions. We have employed a number of computational methods including the complete active space multiconfiguration self-consistent field followed by multireference relativistic configuration interaction computations with spin–orbit coupling that included up to 6 million configurations. We find that the activation barrier to insert uranium into $H2$ is reduced substantially by spin–orbit coupling, and the product species $UH2$ in its $A1$ spin–orbit ground state is substantially stable over $U(5L)+H2$ dissociated products. We have found two electronic states for UH to be quite close to each other, and depending on the level of theory the relative ordering of the $Λ6$ and $I4$ states changes, $I4$ state being the lowest at the highest second-order configuration interaction level. The $UH2$ species also exhibits a similar feature in that the triplet state is favored at the single-reference second-order Møller–Plesset and coupled cluster levels, while the quintet state is favored at the multireference and density functional theory levels. The $UH3$ species is extremely floppy, exhibiting an inversion potential surface that has a barrier smaller than its zero-point energy. It is shown that the $UH3$ species is considerably more ionic than $UH2$ or UH, and $UH3$ is responsible for catalyzing the U-hydriding reaction as the highly positive U site in $UH3$ reacts with $H2$ spontaneously without an activation barrier. The results of our computations are compared with previous experimental results. The spin–orbit coupling is shown to be more important for energy activation than near the minima.