The calculated bond lengths and vibrational frequencies of the octahedral compounds UF6, NpF6, and PuF6 from Hartree-Fock and various density functional calculations are compared. The calculations employ relativistic effective core potentials (RECPs) derived from one-component relativistic Hartree-Fock atomic wave functions. The structures and frequencies of the hexafluoride molecules were obtained using analytic gradient and second derivative techniques. Of the methods examined here the local density (SVWN) and hybrid functional (B3LYP) have the best performance in terms of the errors compared to experiment for the bond lengths and vibrational frequencies. The analytic representations of the RECPs for U, Np, and Pu are given along with Gaussian basis sets for the valence electrons of these actinide elements.