The H2O++ ground state potential energy surface

At the correlation-consistent polarized-valence quadruple-zeta complete act ive space self-consistent field second-order configuration interaction leve l of ab initio theory (cc-pVQZ CASSCF-SOCI), we calculated 129 points on th e ground electronic state potential energy surface of the water dication H2 O++; this calculation includes the energy of (X) over tilde(3)Sigma(-) OHat equilibrium and the energy of the triplet oxygen atom. We determined the parameters in an analytical function that represents this surface out to t he (OH+ + H+) and (O + 2H(+)) dissociation limits, for bending angles from 70 to 180 degrees. There is a metastable minimum in this surface, at an ene rgy of 43 600 cm(-1) above the H+ + OH+ dissociation energy, and the geomet ry at this minimum is linear (D-proportional to h), with an OH bond length of 1.195 Angstrom. On the path to dissociation to H+ + OH+, there is a sadd le point at an energy of 530 cm(-1) above the minimum, and the geometry at the saddle point is linear (C-infinity v) with OH bond lengths of 1.121 and 1.489 Angstrom. Using the stabilization method, we calculated the lowest r esonance on this surface. Relative to the metastable local minimum on the p otential energy surface, the position of the lowest resonance for H2O++, D2 O++, and T2O++ is 1977(85), 1473(25), and 1249(10) cm(-1), respectively, wh ere the width of each resonance (in cm(-1)) is given in parentheses. (C) 19 99 Academic Press.