Density functional studies of UO22+ and AnF(6) (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 relativist ic effective core potentials (RECPs) and extended basis sets. The calculate d 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 experimen tal data for the geometries and vibrational frequencies of the AnF(6) syste ms. The performance of pure gradient-corrected functionals is poor, even wo rse than that of local density functional for the geometries and vibrationa l frequencies. For the vibrational frequencies, the hybrid functional calcu lations give qualitatively the correct order with small quantitative deviat ions from the experimental data. The local density functional provides reli able frequencies for the stretching modes but underestimates the frequencie s for the bending modes. The pure gradient-corrected functionals underestim ate the frequencies for all the stretching and bending modes. While the lar ge-core RECP calculations provide short bond lengths and a rather poor desc ription for vibrational frequencies relative to small-core RECP calculation s for UO22+ and UF6, they may still be a practical choice of method provide d hybrid functionals are also used, especially for larger systems. (C) 2000 American Institute of Physics. [S0021-9606(00)30341-5].