First-principles investigations on C5O5-/0 and C6O6-/0 oxocarbons

► The geometrical and electronic properties of C5O5-/0 and C6O6-/0 oxocarbons have been theoretically investigated. ► AdNDP analyses reveal the bonding patterns of CnOn2-/-/0 oxocarbons (n = 5 and 6). ► The first three VDEs of C5O5- and C6O6- were calculated at various levels.