2 4 8 16 32 R  (a.u.) -20 -16 -12 -8 -4 0 D Z  (a.u.) r =0.8 r =2.4 FIG. 8: The figure shows the D$_{Z}$ component of the permanent electric dipole of H − 3 for one value of γ = 30 ° and several values of r from r =0.8 (the largest value of D$_{Z}$ at small R ) to 2.4 (the smaller value of D$_{Z}$ at small R ). The ab initio values and grid points are indicated by circles. principal axes of inertia for H − 3 and D − 3 are the same, the obtained PDMS are the same for the two species. Figures 8 and 9 show ab initio values of the D$_{Z}$ and D$_{X}$ PDM components for γ = 30 ° for all nine values of r = 0 . 8 , 1 , · · · , 2 . 4 a.u. as a function of R . As one can see from the figures, the sampling grid of ab initio geometries is dense enough to perform an interpolation procedure in order to calculate D$_{Z}$ and D$_{X}$ at any arbitrary geometry. Therefore, in the central region in Fig. 1, we interpolate the D$_{Z}$ and D$_{X}$ PDMS using the same 3D B-spline procedure as for the PES interpolation. In region II, when R → ∞ , the analytical behavior of the largest component D$_{Z}$ is known: It decreases with increasing R as (in a.u.) D$_{Z}$ ≡ er$_{cm-H}$ = − 2 R/ 3, where r$_{cm-H}$ is the distance between the center of mass of H − 3 and the H − ion, and e ≡ 1 is the electron charge. The ab 20