4 8 16 32 R  (a.u.) 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 D X  (a.u.) r =0.8 r =2.4 r =2.2 r =2 r =1.8 r =1.6 r =1.4 r =1.2 r =1 FIG. 9: The D$_{X}$ component of the permanent dipole of H − 3 for γ = 30 ° and different r . The ab initio values and grid points are indicated by circles. initio D$_{Z}$ values confirm this behavior, so the same formula is used for extrapolation of D$_{Z}$ in region II. Outside the central region in Fig. 1, the PDMS (except the D$_{Z}$ components in region II) are extrapolated using an empirical analytical formula. The empirical formula is obtained by inspecting the PDMS variations close to the boundaries of the ab initio region. The smallest component D$_{X}$ varies at large R as k$_{x}$ ( r, γ ) /R , where k$_{x}$ ( r, γ ) is fixed by the value of the PES at the final point R$_{f}$ = 56 . 4227 a.u. of the grid, i.e. k$_{x}$ = V ( R$_{f}$ ; r, γ ) R$_{f}$ . As previously, we obtain a surface for the k$_{x}$ ( r, γ ) function, which is computed at any geometry using a 2D B-spline interpolation. For the extrapolation of the PDMS in region I we found (empirically) a quadratic (for D$_{Z}$ ) and linear (for D$_{X}$ ) dependencies along r with coefficients depending on R and γ , which are evaluated from the corresponding boundary values of D$_{Z}$ and D$_{X}$ respectively. Similarly, in region III and IV, we assumed a linear dependency along r and R respectively, with coefficients fixed by the boundary values of the PDMS. 21