Abstract: The equilibrium structures, the charge population, and the  spectroscopic properties of UO, UO$_{2}$, UO$_{3}$, and U$_{2}$O$_{3}$ molecules are systematically investigated using the density functional theory (DFT)  with the method of generalized gradient approximation (GGA). The bond lengths and the vibrational frequencies of the ground states of UO, UO$_{2 }$, and UO$_{3 }$ molecules are all in agreement with available experimental  data. For U$_{2}$O$_{3 }$ molecules, our calculations indicate that the ground state of the  U$_{2}$O$_{3}$ molecule is an $\tilde {\rm X}^{7}$A$_{1}^\prime$ state with $D_{3h }$ (trigonal bipyramid) symmetry  ($R_{1}$(U--O)=0.2113 nm, $R_{2}$(U$_{1}$--U$_{2}$)=0.2921 nm, $\angle$U$_{1}$OU$_{2}=87.5$$^\circ$ , dihedral angle $\varTheta $(U,O$_1$,O$_2$,O$_3$)=62.40$^\circ$). The harmonic frequency, the  IR intensity and the spin  density of the U$_{2}$O$_{3}$ molecule are all obtained for the first time in theory. For the ground state of U$_{2}$O$_{3 }$ molecules, the vibrational frequencies are 178.46 (A$_{1}^\prime$), 276.79 (E$_{1}''$), 310.77  (E$_{1}^\prime$), 396.63 (A$_{2}''$), 579.15 (E$_{1}^\prime$), and 614.98 (A$_{1}^\prime$) cm$^{ - 1}$. The vibrational modes corresponding to the IR maximum peaks are worked out for UO$_{3}$ and U$_{2}$O$_{3 }$ molecules.  Besides, the results of Gophinatan--Jug bond order indicate that UO, UO$_{2 }$, and UO$_{3 }$ molecules possess U=O double bonds and that the U$_{2}$O$_{3}$ molecule possesses U--O single bonds and a U--U single bond.

Key words: uranium oxides molecule, molecular geometry, charge population, vibrational spectrum