Ground and excited states of HNC, NH, and NH2 transients: Ab initio geometries, electronic structures, and molecular properties

Geometrical and vibrational characterization of NH2((X) over tilde B-2(1),( A) over tilde (2)A(1),(B) over tilde B-2(2)), NH((X) over tilde (3)Sigma (- ),(a) over tilde (1)Delta, (b) over tilde (1)Sigma (+),(A) over tilde (3)Pi ,(c) over tilde (1)Pi,(d) over tilde (1)Sigma (+)), HNC((X) over tilde (1)S igma (+)) and hitherto experimentally unreported HNC((a) over tilde (3)A',( b) over tilde (3)A ") systems is reported by taking different sets of activ e electrons/orbitals in complete active space self consistent field (CASSCF ) calculations. Employing CASSCF optimized geometries, the excitation energ ies, electric field gradients, and dipole moments for these systems have be en computed at complete active space second-order perturbation (CASPT2) and multireference singles and doubles configuration interaction (MRD-CI) leve ls of theory. Computed field gradients are very similar at CASSCF and CASPT 2 levels, but differ significantly with those obtained at MRD-CI level. The outcome of present numerical experiment is that the inclusion of excitatio ns higher than singles and doubles and use of a very large CI space employi ng iterative natural orbitals is necessary to account for the anisotropy of the molecular charge distribution around any nucleus in a molecule. (C) 20 01 American Institute of Physics.