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The ground state potential energy surfaces of MgNC and CaNC were calculated by highly correlated {ab initio} single-reference Averaged Coupled Pair Functional method. The basis sets used are [5s3p2d1f+lslp] for C and N, [6s5p31f] for Mg and [10sl0p6d2f] for Ca respectively. Mg L-shell core-valence correlation and Ca M-shell core-core correlation were included for MgNC and CaNC, respectively. Both species are considered as ionic compound formed of $M+$ and $CN-$ in their ground $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +$ electronic states Therefore the directionalities of metal-NC bonding in and MgNC and CaNC are very small and potential energy surfaces for bending motion are extremely shallow, Theoretically calculated harmonic frequencies of bending mode are $90cm-1$ for MaNC and $60cm-1$ for CaNC, and the barriers of isomerization to metal-stable MgCN and CaCN are $2160cm-1$ and $1950cm-1$, respectively. Although two molecules are isovalent, the shapes of their bending potential are quite different. The bending potential for MgNC is essentially of harmonic nature, while that for CaNC has quite anharmonic and anisatroopie character (Fig. 1). For CaNC, the first excited state of $\nu 2$ mode in harmonic approximation reaches the strong anharmonic region of bending potential. Furthermore, the second excited state in harmonic approximation reaches the region where bending angle strongly couples with the CaN bond length (Fig.2). In the region where bending angle is smaller than l20 degree, the CaN bond length becomes longer than the value at linaer equilibrium geometry. Such difference in bending potential between MgNC and CaNC is one of the reason why centrifugal distortion constants up to tenth order are needed to interpret the microwave spectrum of $CaNC1$ while only while only within sixth order terms are necessary for $MgNC.2$ [FIGURE]