The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, CNN, have been predicted using the variational method. The potential energy surfaces of CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multi-reference complete active space self-consistent-field (CASSCF), and internally contracted multi-reference configuration interaction (ICMRCI) methods. The correlation-consistent polarised valence quadruple zeta (cc-pVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laser-induced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both non-dynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the cc-pVQZ ICMRCI level of theory, ν1 = 1230, ν2 = 394, and ν3 = 1420 cm− 1, are in excellent agreement with the experimental values of ν1 = 1235, ν2 = 396, and ν3 = 1419 cm− 1, and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm− 1. It is shown that the previously suggested observation of the ν3 frequency at about 2847 cm− 1 was in fact the first overtone 2ν3.