Ab InitioPrediction of the Rotation-Vibration Spectrum ofH3+andD3+

The first few lines in the $P$, $Q$, and $R$ branches of the rotation-vibration spectra of the equilateral-triangle molecular ions ${\mathrm{H}}_3^{+}$ and ${\mathrm{D}}_3^{+}$, obtained by ab initio nonperturbative calculations, are reported. Comparison with observations indicates an accuracy better than 1% was obtained for both the infrared-active fundamental vibration frequency and the equilibrium internuclear distances.