It is easy to rationalize why there are two nearly degenerate triplet states. At the dissociation limit, the O( 3 P) term comprises 9 triplet states (3 p-states for each S z ). From these, one porbital (call it p z (O)) can form a bonding combination with the π-orbitals of ethylene, while others yield non-bonding combinations. In the triplet state with doubly occupied p z (O), an antibonding orbital is occupied leading to repulsive state, while in two other triplets, differing in occupation of non-bonding lone pairs, there is bonding interaction between O and C (as clearly illustrated by the natural orbitals in Fig. of Ref. 7). The singlet A states can also contain closed-shell configurations. The weight of the closed-shell configuration in the lowest singlet is comparable with the weights of open-shell configurations. This closed-shell configuration leads to bonding interactions between the two radical centers. The weight of this configuration and, consequently, bonding interactions increase at smaller CCO angles, leading to an important product, ethylene and propylene oxides.