An ab initio molecular orbital study of the nuclear volume effects in uranium isotope fractionations

This paper discusses the nuclear volume dependence of uranium isotope fractionations in the U3+-U4+ and U4+-UO22+ systems by reference to a series of ab initio molecular orbital calculations. Nuclear volume-dependent terms ( ln K-nv) in isotope fractionation coefficients (epsilon) are calculated from the energetic balance of the isotopomers involved in the systems. We used the Dirac-Coulomb Hartree-Fock (DCHF) method with the Gaussian-type finite-nucleus model. We employed three types of generally contracted Gaussian basis sets to check the basis set dependences. In the U3+-U4+ system, the present values of ln K-nv for uranium, other than those with the smallest double-zeta basis set, are in good agreement with previous values of ln K-nv obtained from a numerical atomic multiconfigurational DCHF method with the Fermi-type finite-nucleus model. The present calculations reasonably reproduce the experimental value of epsilon in the U3+-U4+ system, and the value of ln K-nv in the U4+-UO22+ system, obtained empirically by temperature-dependent fitting of the experimental epsilon values. For instance, in the U4+-UO22+ system, the present ab initio ln K-nv value for a U-235-U-238 isotope pair is 0.002 09 using the largest basis set, while the experimental value is 0.002 24. This paper also shows that nuclear volume effects are negligibly small on the U-O bond length and two force constants of UO22+. Hence, the molecular vibrational terms of the isotope fractionation coefficients mainly depend on the nuclear mass rather than the nuclear volume. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2992616]