A relativistic density functional study on the uranium hexafluoride and plutonium hexafluoride monomer and dimer species

A study on the UF₆ monomer and dimer was carried out within the density functional method. The U-F distance in the UF₆ monomer was optimized at different levels of theory, pointwise, assuming octahedral geometry, (1) by using an all-electron basis for both U and F in a nonrelativistic calculation; (2) by using a relativistic effective core potential (RECP) on U and nonrelativistic effective core potential (ECP) on the fluorines; and (3) by using RECP on the U atom and an all-electron basis on the F atoms. Atomization energies of 23.11, 33.92, and 35.66 eV were obtained at the three levels, respectively. Relativistic effects lead to about a 50% increase in the atomization energy. For the UF₆ dimer, the potential energy curve, as a function of the intermolecular U-U distance, was computed at level 2, and the rotational barrier between the two monomers was determined. Similar calculations were performed on the corresponding PuF₆ species. Comparisons are made with experiment and other theoretical studies, where available.