**
Natural orbitals of parent carbonyl oxide
calculated at MP2/aug-cc-PVDZ level.**

Used software (calculation/visualization): MOLPRO and MOLDEN

**
Molecular orbitals of parent carbonyl oxide
calculated at RHF/6-31G* level.**

Used software (calculation/visualization): Gaussian 98 and WinMopac

**
Molecular orbitals of parent carbonyl oxide
calculated at MINDO/3 level.**

Used software (calculation/visualization): ViewMol3D 4.33.6