AB-INITIO POTENTIAL-ENERGY SURFACE FOR THE COLLISIONAL SYSTEM H--2 AND PROPERTIES OF ITS VAN-DER-WAALS COMPLEX(H)

An accurate ab initio potential energy surface for the reaction H-2+H- has been determined by MR-CI and CEPA(2) calculations for 403 nuclear configurations. It is cast in analytical form by a fit with 23 parame ters and a root-mean-square error of 0.27 mE(h) (0.07 mE(h) for all po ints below 50 mE(h)). The position and the height of the barrier have been found as r(b) = 1.997 bohr and 16.80+/-0.2 mE(h) (10.55+/-0.15 kc al/mol), respectively. Taking the change in zero-point energy into acc ount, a threshold energy for the exchange reaction D2 + H- --> HD+D- o f 0.49 eV is obtained, in good agreement with the experimental value o f 0.42+/-0.12 eV. A linear van der Waals minimum has been found at r = 1.416 bohr and R = 6.183 bohr with a well depth of 1.75 mE(h) (1.10 k cal/mol). An accurate treatment of the zero-point motion leads to the new result that the H-3- complex is stable thermodynamically. The pote ntial supports four bound vibrational levels. The electron detachment seam has been determined and its lowest point is found for R = 2.87 bo hr, r = 1.42 bohr and gamma = 90-degrees with an energy of 1.2 eV, whi ch again agrees comfortably with the experimental threshold of 1.2+/-0 .1 eV for H-2 + D-.