N₃ ⁺: Full-dimensional ground state potential energy surface, vibrational energy levels, and dynamics

The fundamental vibrational frequencies and higher vibrationally excited states for the N₃ ⁺ ion in its electronic ground state have been determined from quantum bound state calculations on three-dimensional potential energy surfaces (PESs) computed at the coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ-f12 and multireference configuration interaction singles and doubles with quadruples (MRCISD+Q)/aug-cc-pVTZ levels of theory. The vibrational fundamental frequencies are 1130 cm^-1 (ν₁, symmetric stretch), 807 cm^-1 (ν₃, asymmetric stretch), and 406 cm^-1 (ν₂, bend) on the higher-quality CCSD(T)-F12b surface. Bound state calculations based on even higher level PESs [CCSD(T)-F12b/aug-cc-pVQZ-f12 and MRCISD+Q-F12b/aug-cc-pVTZ-f12] confirm the symmetric stretch fundamental frequency as ∼1130 cm^-1. This compares with an estimated frequency from experiment at 1170 cm^-1 and previous calculations [Chambaud et al., Chem. Phys. Lett. 231, 9-12 (1994)] at 1190 cm^-1. The remaining disagreement with the experimental frequency is attributed to uncertainties associated with the widths and positions of the experimental photoelectron peaks. Analysis of the reference complete active space self-consistent field wave function for the MRCISD+Q calculations provides deeper insight into the shape of the PES and lends support for the reliability of the Hartree-Fock reference wave function for the coupled cluster calculations. According to this, N₃ ⁺ has a mainly single reference character in all low-energy regions of its electronic ground state (³A″) PES.