levels of H − 3 and D − 3 and Section V presents our results on the dipole moment of H − 3 . Section VI is the conclusion. Atomic units (a.u.) for distances (1 a.u. = 0.0529177 nm) and for energies (1 a.u. = 219474.63137 cm − $^{1}$) will be used throughout the paper, except otherwise stated. As in Ref. [10], we used the coupled-electron pair approximation (CEPA-2) method [49], which is part of the Molpro package [50]. It is a non-variational variant of the configuration interaction method for closed-shell molecules. Here we used a considerably larger basis set, AV5Z with spdfg basis functions from the Molpro basis library, and a much larger number of geometries, than in Refs [10, 11]. As H − 3 is a van der Waals molecule, we defined a 3D-grid in Jacobi coordinates: r – the distance between two protons, R – the distance from the center of mass of the two protons to the third proton, and γ – the angle between vectors ⃗ R and ⃗ r . The grid in r is uniform from r = 0 . 8 a.u. to r = 2 . 4 a.u. and changes by a step Δ r = 0 . 2 a.u. The grid in γ changes from 0 to 90 ° by a constant step of Δ γ =15 ° . The grid in R was chosen denser for small R than for large R : the grid points R$_{i}$ were calculated according to R$_{i}$ = 1 . 5 + 0 . 452 exp( i/ 10) (in a.u.) with i = 1 , 2 , · · · , 48, which makes R changing from 1.9995 a.u. to 56.4227 a.u. Therefore, the calculations were performed for 9 × 7 × 48 = 3024 geometries. Notice that the above grid starts at a smaller value of r = 0 . 8 a.u. than in Refs. [10, 11] ( r = 1 a.u.). Indeed, we found that a grid starting at r = 1 cannot properly represent the repulsive part of the H$_{2}$ ground state potential curve (at fixed R and γ ) and gives an appreciable error in energies obtained of the lowest H$_{2}$ vibrational levels and, as a result, a comparable error in dissociation energies of H − 3 → H$_{2}$( v, j )+H − when R → ∞ . We also used a denser grid in γ and a longer and denser grid in R than in Refs. [10, 11]. For each geometry we calculated the potential energy of the H − 3 ground state, and the components of the PDM vector with respect to the principal axes of inertia. The obtained ab initio PES and PDMS were used to prepare Fortran subroutines that cal- culate PES and PDMS for any arbitrary geometry. Inside the three-dimensional box r ∈ [0 . 8; 2 . 4] , R ∈ [1 . 9995; 56 . 4227], γ ∈ [0; 360 ° ] (central region in Fig. 1) the proce- dures interpolate the surfaces using the three-dimensional B-spline method. Outside of the 4 II. AB INITIO CALCULATION AND INTERPOLATION OF THE H − 3 GROUND STATE POTENTIAL SURFACE