Ab initio quartic force fields for anions: A benchmark study on OH−16, OH−18, and OD−16

The singles and doubles coupled-cluster method that includes a perturbational correction for connected triple excitations, denoted CCSD(T), is used in conjunction with several one-particle basis sets to determine quartic force fields for ${\mathrm{OH}}^{\mathrm{-}}.$ Basis set convergence properties of the various rovibrational spectroscopic constants is studied. To assure numerical stability of the quartic force fields and to investigate variational calculations of vibrational energy levels, sextic force fields have also been evaluated. The largest one-particle basis set employed contains up through $h$ functions on oxygen and $g$ functions on hydrogen. Agreement between experiment and theory is excellent and it is shown that the $\mathrm{CCSD(T)}\mathrm{/spdfg}$ approach performs as well for ${\mathrm{OH}}^{\mathrm{-}}$ as it previously did for ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}.$ Due to the inherent difficulty in high resolution studies on molecular anions, it is concluded that currently the $\mathrm{CCSD(T)}\mathrm{/spdfg}$ approach may be the most reliable technique for the determination of accurate rovibrational spectroscopic properties of small- to medium-sized anions.