Spectroscopic constants of the two lowest states of the InN molecule, the X-3 Sigma(-) ground and the (3)Pi excited state were calculated using the restricted open-shell Hartree-Fock Coupled Cluster ROHF-CCSD(T) method with single, double and perturbative triples, the complete active space second-order perturbation theory (CASPT2) and the multireference configuration interaction (MRDCI) methods. Relativistic pseudopotentials (for MRDCI) and atomic natural orbital basis set for treating spin-orbit and scalar relativistic effects - ANORCC (for CCSD(T), CASPT2 and CASPT2/RASSI-SO) were used. The accuracy of different methods was compared correlating up to 26 valence and core-valence electrons of N and In atoms. (C) 2007 Elsevier B.V. All rights reserved.