Theoretical Study On Properities Of Uranium Oxide

The first ionization potentials (IPs) of U, UO and UO2, as well as the optimized structures and vibrational frequencies of UO, UO22+ and UO3 from Hartree-Fock and various density functional calculations are compared with theortical and experimental datas. Within the latter approach, generalized gradient approximation [(GGA): BLYP, BPW91], and hybrid functional (B3LYP and B3PW91) were considered. The hybrid functional is a three-parameter mixture of density functional and"exact"HF exchange. The calculations employ relativistic effective core potentials (RECPs) derived from one-component relativistic Hartree-Fock atomic wave function. In all the calculations the basis set used for O was a double zeta basis with polarization and diffuse functions, 6-311+G*. For the U atom energy-adjusted Large-core (LC: 14 valence electrons) RECPs and small core (SC: 32 valence electrons) was used, the corresponding (8s8p6d5f2g)/[5s5p4d3f2g] basis set and (12s11p10d8f) /[8s7p6d4f]were used in the calculations respectively. The convergence criteria scf=tight (energy and density matrix convergence 10-8 au) and interaction grid option grid=ultrafine(99 radia shells and 590 angular point per shell) were used to ensure good numerical accuracy.For U atom we compared the first IP with two different basis set and saw that the B3LYP method with SC-RECP got a excellent agreement with experimental data. For UO and UO2 also got good results compared with expeimental studies. For the structures and vibrational frequencies of other uranium compounds, hybrid density functional methods all obtained a better results compared with HF and pure GGA.In order to investigate the influence of the 5f electrons to actinide compounds, we reoptimized the structure of uranium compounds with the basis set of which we removal the f conponent out and found the changes in the structure and vibrational frequencies obviously. The structure usually be a saddle point not a minimum. For UO3 the 5f orbital have greater contribution to the two"equal-bond"than the lone U-O bond, and caused the symmetry changed from C 2v to D3 h.