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Potential energy curves of 14 electronic states of yttrium nitride (YN) are calculated by using multistate complete active space second order perturbation theory. The ground state is predicted as X 1 R þ and has the equilibrium bond length of 1.809 Å and vibrational frequency of 658 cm À1 . Two low-lying excited states, a 3 R þ and A 1 R þ , locate at 2,360 and 3,670 cm À1 higher than the ground state, which are in agreement with experimental excitation energies of 2,423 and 3,700 cm À1 , respectively. The high-order singlet, the D 1 P state, lies at 19,232 cm À1 above the ground state with the equilibrium bond length of 2.057 Å and vibrational frequency of 562 cm À1 . TheR transition is dipole allowed and may contribute to spectra of YN in visible region.

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