Ground and valence excited states of C2N and CN2 transients: Ab initio geometries, electronic structures, and molecular properties

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)P i, 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 electro nic states with CASSCF potentials. Excitation energies with zero-point corr ections, To, electric field gradient (efg), and dipole moment, mu, are calc ulated 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 com ponents V-XX, V-YY, and V-ZZ of the efg tensor listed through two quantitie s eq(1)(= V-ZZ) and eq(2)(= V-XX-V-YY) are not very different from their CA SPT2 counterparts, suggests that second-order perturbation involving all si ngles and doubles over the one-dimensional space spanned by the CASSCF wave function are not important for the efg and mu. However, the important cont ributions come from the higher excitations (triple, quadruples, etc.), whic h are included in MRD-CI wave function, by taking multireference zeroth-ord er wave function. The use of iterative natural orbital seems to be necessar y to obtain stable values of the efg and mu in the MRD-CI method. (C) 2001 American Institute of Physics.