Besides the fundamental importance, compounds containing C, N and O have great potential as the so-called “high-energy density materials” (HEDMs). Due to the explosive nature, a HEDM usually has great difficulty in laboratory synthesis and characterization. Thus, reliably finding a HEDM candidate with good kinetic stability (i.e., rate-determining barrier is above 20 kcal/mol) in computation has proven to be the key base for the subsequent experimental study, as well documented by two recent examples, i.e., diazirinone ([C,N2,O]-A, Angew. Chem. Int. Ed. 2011, 50, 1720) and nitryl cyanide ([C,N2,O2]-B, Angew. Chem. Int. Ed. 2014, 53, 6893). What is the next synthetic target for molecular HEDMs? Having been predicted to possess both large exothermicity (190 kcal/mol) and large decomposition barrier to N2+CO2 (29 kcal/mol) at the CCSD(T)/TZ2P//MP2/6-31G(d) level (J. Phys. Chem., 1996, 100, 19840), C with a bicyclic [C,N2,O2] structure, appears to well deserve the synthetic trial for HEDMs. In this work, by re-evaluating the stability of C, we located a new rate-determining transition state for decomposition (TS2). With TS2, a significantly reduced decomposition barrier was arrived at, i.e., 11.6-13.0 kcal/mol at the G3B3, CBS-QB3, G4, W1BD and CCSD(T)_CBS//B3LYP/aug-cc-pVTZ composite energy calculational levels using the restricted wave function. Strikingly, TS2 possesses significant open-shell feature and the barrier was further reduced to be 6.9 kcal/mol at the UCCSD(T)_CBS//UB3LYP/aug-cc-pVTZ level. Thus, the bicyclic isomer C is unlikely to be a molecular HEDM, though its spectroscopic detection could still be feasible. Finally, the ring-opening stability of c-CO2 moiety in the present [C,N2,O2]-C largely contrasts with that in the well-known methyldioxirane, demonstrating the different influence on the complexation to c-CO2.