Density functional studies of UO22+ and AnF6 (An=U, Np, and Pu) using scalar-relativistic effective core potentials

We have performed one-component relativistic calculations to determine the bond lengths and harmonic vibrational frequencies of UO22+ and AnF(6) (An=U, Np, and Pu) using various density functional calculations with relativistic effective core potentials (RECPs) and extended basis sets. The calculated spectroscopic constants using small-core RECPs compare favorably with the available all-electron relativistic results for UO22+ and UF6. The results using the hybrid functionals are in very good agreement with the experimental data for the geometries and vibrational frequencies of the AnF(6) systems. The performance of pure gradient-corrected functionals is poor, even worse than that of local density functional for the geometries and vibrational frequencies. For the vibrational frequencies, the hybrid functional calculations give qualitatively the correct order with small quantitative deviations from the experimental data. The local density functional provides reliable frequencies for the stretching modes but underestimates the frequencies for the bending modes. The pure gradient-corrected functionals underestimate the frequencies for all the stretching and bending modes. While the large-core RECP calculations provide short bond lengths and a rather poor description for vibrational frequencies relative to small-core RECP calculations for UO22+ and UF6, they may still be a practical choice of method provided hybrid functionals are also used, especially for larger systems. (C) 2000 American Institute of Physics. [S0021-9606(00)30341-5].