Full Configuration Interaction Energies, Geometries, and Quartic Force Fields of the Nitrenium Ion

Abstract

Benchmark configuration interaction energies, geometries, dipole moments, and harmonic vibrational frequencies are obtained for four low-lying electronic states of NH2+ and for X2B1 NH2 by solving the electronic Schrodinger equation exactly within a double-ζ plus polarization (DZP) basis set and restricting the nitrogen 1s-like core orbital to remain doubly occupied. In addition, full quartic force fields have been determined, and sets of anharmonic spectroscopic constants and fundamental frequencies are reported for the ã 1A1 state of NH2+. Vertical and adiabatic ionization potentials of NH2 are also determined exactly within a DZP basis. The capability of less-complete electron correlation treatments to match these exact, full configuration interaction (full CI) results is assessed. The efficacy of obtaining anharmonic force fields at nonstationary geometries is also examined, and in agreement with previous work, it is found that the self-consistent field method can provide high quality cubic and quartic force constants when they are evaluated at the full CI equilibrium geometry. Both the CCSD(T) and CASSCF-SOCI methods provide geometric and spectroscopic data in excellent agreement with the Full CI results and are competitive in their reliability with more expensive procedures (e.g. CISDTQ). The second 1A1 state in C2v symmetry is predicted by the TZ2P(f,d) CASSCF-SOCI wavefunction to be linear (thus properly labeled as c1Σg+), even though it is found to be quasilinear at the DZP full CI level.