Abstract: The geometries, energetics and spectroscopic properties of oxygen clusters, Oxy(x=2~6, y=-2~2), were investigated at the B3LYP/6-311G (d, p) level. The CASSCF calculations were carried out for the ground and excited states of3O2and2O2+. The total energy is3O2(3Σg-)<2O2-(2Πgi)<1O2(1Δg)<1O2-2(1Σg+)<2O2+(2Πg)<1O2+2(1Σg+). The relative energy of the active doublet anion of oxygen molecule,2O2-(2Πgi), is only 28 kJ/mol higher than the triplet neutral oxygen molecule,3O2(3Σg-). The calculated O-O vibrational frequencies all are in good agreement with the experimental values. They are 1577 (1580), 1139 (1090), 1563 (1484), 627 (615~545) and 1993 (1905) cm-1, where the O-O vibrational frequency values in parentheses are experimental values, for3O2(3Σg-),2O2-(2Πgi),1O2(1Δg),1O2-2(1Σg+) and2O2+(2Πg), respectively. Moreover, the O-O vibrational frequency of1O2+2(1Σg+) was computed as 2368 cm-1which has not been reported before at both experimental and theoretical levels. Both bent and linear geometries of O3were studied. The bent-types of O3are more favorable than the linear-type in energy. Three types of structure for oxygen trimers are calculated at the B3LYP/6-311G (d, p) level. They are the structure-I with an obtuse angle of O-O-O,the structure-II with an acute angle of O-O-O, and the structure-III of linear type. For a bent-type structure of O3species (structure-I), the total enegy is2O3-(2B1)<1O3(1A1)<3O3(3B2)<1O3-2(1A1)<2O3+(2A1). The optimization of geometry at B3LYP/6-311G (d, p) level indicated that the species of2O3-(2B1) with 1.3573 of O-O bond length and 115.6584o of O-O-O bond anger is the ground state of O3. The total energy of O4species and their ions is2O4-(Cs,2A′, bend-type)<2O4-(C2v,2A2,face-centered triangle-type)<2O4-(D∞h,2Σg, linear-type)<1O4(Cs,1A′, bend-type)<1O4(D∞h,1Σg, linear-type)<1O4(D4h,1A1g, square-type)<1O4(C2v,1A1, face-centered triangle-type)<2O4-(D4h,1A1g, square-type)<2O4+(D∞h,2Σg, linear-type)<2O4+(Cs,1A′, bend-type). The species with the lowest relative energy is an anion,2O4-(Cs,2A′, bendtype), with chair form geometry and characteristic vibronic frequencies of 1179 and 1349 cm-1. The relative energy of1O5(C2v,1A1) with coplanar-triangle-bicone geometry is the lowest among the O5species and their ions, which may be a resonance structure with1O5(C2v,1A1) of A type. Their characteristic vibronic frequency is 1302 cm-1. The relative energy of the O6species and their ions with hexagon geometry is lower than one with linear geometry. Their infrared vi-bronic intensity may be weak and unobservable but the Raman vibronic intensity may be strong and observable based on their symmetry.