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The A ( r, γ ) and B ( r, γ ) coefficients are obtained in a way similar to the one described above, considering the first two values of the ab initio potential energy V ( R = 2 a.u.; r, γ ) and V ( R = 2 . 052 a.u. ; r, γ ), and are further interpolated for any ( r , γ ) using the same 2D B-spline procedure. 

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We show in Fig. 2 the H − 3 PES as a function of two internuclear distances, r$_{1}$ and r$_{2}$ for two values (180 and 90 ° ) of the bonding angle δ . These coordinates are convenient to describe main features of the PES. As expected, the figure is symmetric with respect to the r$_{1}$ ↔ r$_{2}$ exchange, and shows the van der Waals potential well in each coordinate (with the minimum labeled with M ), separated by a potential barrier (with the maximum labeled with B ) for the exchange of two identical nuclei. As in previous studies, lowest energy is found for a linear configuration. The position and energy of M and B are reported in Table I, together with other characteristic parameters of the PES. 

The Fortran subroutines calculating the PES and PDMS from the present ab initio data 

