The molecular structures and spectra for PuO₃ are worked out by ab initio HF, and density functional B3LYP and BHANDHLYP methods with relativistic effective core potential (RECP) for Pu atom and 6-31+g^* for O atom. The effects of symmetry C_2v, D_3h and C_3v for the four atoms PuO₃ molecule, the calculation levels on molecular energies, and equilibrium structures have been evaluated. The results show that the neutral PuO₃ molecule could exist stably in C_2v structure (B3LYP: R₁=0.19143 nm, R₂(=R₃)=0.22057 nm, ∠OPuO=102.2108°), its ground electronic state is ⁷B₁, the D_3h structure could be assigned to a metastable state ⁷A₂″, and there would be no stable C_3v structure. The corresponding vibrational harmonic frequencies, vibrational infrared intensities, Raman activities, Raman depolarizabilities, Mulliken charge populations, spin densities, polarizabilities and electric dipole moments have also been tabled out at first time. The same calculations performed on PuO₃⁺ ion also show that the C_2v structure could exist stably, and its ground state is ⁶B₂.

Key words: PuO₃, PuO₃⁺, density functional theory, relativistic effective core potential(RECP), molecular structure and molecular spectra