Ab initio potential energy surface for linear H₃

The configuration interaction (CI) method has been applied to determine an accurate potential energy surface for linear Hj. The calculated surface is believed to lie no more than 0.8 kcal/mole and no less than 0.2 kcal/mole above the exact surface, a significant improvement over all previous calculations. The calculation yields a linear symmetric saddle point at an internuclear separation of 1.757 a.u. The saddle point energy is 9.8 kcal/mole above the calculated energy of an isolated hydrogen atom and a hydrogen molecule, and 10.28 kcal/mole above the exact energy of the isolated systems. An estimated lower bound for the barrier height is 9.5 kcal/mole. Comparison of the calculated Hj potential surface with the best semiempirical surface and the best previous ab initia surface shows good qualitative agreement. However, there are quantitative differences, whose effect can only be determined by accurate dynamical calculations. The method employed in the Ha calculation was designed to approach the complete CI result with a severely truncated CI expansion. It attempts to establish, computationally, convergence patterns that can be reliably extrapolated to the complete CI limit. Thus, the calculations reported here also serve the purpose of evaluating a computational method applicable to more complicated systems.