Calculations of the vibrational frequency and isotopic shift of UF₆ and U₂F₆

Molecular structure, vibrational frequency and infrared intensity of UF₆ are investigated by using revised Perdew--Burke--Enzerhof function with triple-zeta polarized basis set. The calculation results are in good agreement with the experimental values and indicate the existence of stable U₂F₆ molecule with a multiply bonded U₂ unit. The calculation results also predict that the D_3d symmetry of U₂F₆ is more stable than D_3h. The optimized geometries, vibrational frequencies, and infrared intensities are also reported for U₂F₆ molecules in D_3d symmetry. In addition, the isotopic shift of vibrational frequencies of the two molecules under isotopic substitution of uranium atom are also investigated with the same method. The U₂F₆ molecule is predicted to be better than UF₆ for laser uranic isotope separation.