In order to identify the ground state of CoN, we carried out calculations of multi-reference single and double excitation configuration interaction with Davidson-type correction and multi-reference coupled pair approximation (MRCPA). MRCPA predicts that (1)Sigma(+) is the ground state of CoN. The (5)Delta state lies only 0.223 eV above (1)Sigma(+). Inter-nuclear equilibrium distance, harmonic frequency, and rotational constants of (1)Sigma(+) agree well with experimental results. (C) 2003 Elsevier B.V. All rights reserved.