The potential energy surfaces of CaNC ((X) over tilde (2)Sigma(+)) and CaCN ((X) over tilde (2)Sigma(+)) have been investigated by the highly correlated ab initio molecular orbital methods. The bending potential for CaNC is shallow, and shows quite anharmonic and anomalous character, which can explain why the centrifugal distortion constants up to the tenth order were required for the analysis of its rotational spectrum. The reaction path for the isomerization reaction of CaNC and CaCN was also determined: The activation barrier is 2111 cm(-1) from the CaNC side, and 602 cm(-1) from the CaCN side. Core-core and core-valence correlation contributions of Ca M-shell electrons make the Ca-N (for CaNC) and Ca-C (for CaCN) bond lengths shorter by 0.05 and 0.04 Angstrom, respectively, which indicates the significance of these core-correlation effects. (C) 2003 Elsevier Science B.V. All rights reserved.