The ground ((X) over tilde (3)Sigma(-)) and first excited triplet ((A) over tilde (3)Pi) electronic states of diazocarbene (CNN) have been investigated systematically starting from the self-consistent-field theory and proceeding to the coupled cluster with single, double, and full triple excitations (CCSDT) method with a wide range of basis sets. While the linear (X) over tilde (3)Sigma(-) ground state of CNN has a real degenerate bending vibrational frequency, the (A) over tilde (3)Pi state of CNN is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequencies of the (A) over tilde (3)Pi state were evaluated via the equation-of-motion coupled cluster (EOM-CC) techniques. The significant sensitivity to level of theory in predicting the ground-state geometry, harmonic vibrational frequencies, and associated infrared intensities has been attributed to the fact that the reference wave function is strongly perturbed by the excitations of 1pi-->3pi followed by a spin flip. At the highest level of theory with the largest basis set, correlation-consistent polarized valence quadruple zeta (cc-pVQZ) CCSDT, the classical (X) over tilde-(A) over tilde splitting (T-e value) was predicted to be 68.5 kcal/mol (2.97 eV, 24 000 cm(-1)) and the quantum mechanical splitting (T-0 value) to be 69.7 kcal/mol (3.02 eV, 24 400 cm-1), which are in excellent agreement with the experimental T-0 values, 67.5-68.2 kcal/mol (2.93-2.96 eV, 23 600-23 900 cm(-1)). With the EOM-CCSD method the Renner parameter (epsilon) and averaged bending vibrational frequency ((ω) over tilde (2)) for the (A) over tilde (3)Pi state were evaluated to be epsilon=-0.118 and (ω) over tilde (2)=615 cm(-1), respectively. They are in fair agreement with the experimental values of epsilon=-0.07 and (ν) over tilde (2)=525 cm(-1). (C) 2004 American Institute of Physics.