Carbon trioxide has been studied using DFT and TDDFT methods with different combinations of GGA functionals and correlation consistent basis sets. The C 2v isomer is calculated to be 2.5kcalmol −1 and 12.5kcalmol −1 lower in energy than D 3h with a conversion energy barrier of 5.0kcalmol −1 and 12.9kcalmol −1 by PW91PW91 and B3LYP, respectively. All of the above include zero-point energy corrections. The geometry changes almost linearly along the reaction coordinate and the TS structure is approximately an average of D 3h and C 2v with an imaginary O–C–O bending mode of 421i cm −1 . However, D 3h is more favored in low energy excited singlet states. These states also promote the C 2v →D 3h conversion.