Geometric and vibrational characterization of CCN((X) over tilde (2)Pi,(a) over tilde (4)Sigma (-),(A) over tilde (2)Delta-,(B) over tilde (2)Sigma (-),(C) over tilde (2)Sigma (+)), CNC((X) over tilde (2)Pi (g),(A) over tilde (2)Delta (u),(B) over tilde (2)Sigma (-)(u)), CNN((X) over tilde (3)Sigma (-),(a) over tilde (1)Delta,(b) over tilde (1)Sigma (+),(A) over tilde (3)Pi, 1 (1)Pi) and NCN((X) over tilde (3)Sigma (-)(g),(a) over tilde (1)Delta (g), (b) over tilde (1)Sigma (+)(g),(A) over tilde (3)Pi (u)) systems have been done using full-valence complete active space SCF (CASSCF) method. The Renner-Teller interaction parameter, epsilon, is calculated for Pi electronic states with CASSCF potentials. Excitation energies with zero-point corrections, To, electric field gradient (efg), and dipole moment, mu, are calculated using CASSCF, complete active space second order perturbation theory (CASPT2) and multireference singles and doubles configuration interaction (MRD-CI) levels of theory. The fact that CASSCF values of the principal components V-XX, V-YY, and V-ZZ of the efg tensor listed through two quantities eq(1)(= V-ZZ) and eq(2)(= V-XX-V-YY) are not very different from their CASPT2 counterparts, suggests that second-order perturbation involving all singles and doubles over the one-dimensional space spanned by the CASSCF wave function are not important for the efg and mu. However, the important contributions come from the higher excitations (triple, quadruples, etc.), which are included in MRD-CI wave function, by taking multireference zeroth-order wave function. The use of iterative natural orbital seems to be necessary to obtain stable values of the efg and mu in the MRD-CI method. (C) 2001 American Institute of Physics.