Ab Initio Studies of Small Molecules Using 1s Gaussian Basis Functions. II. H3+

Abstract

Self-consistent-field and CI variational calculations are presented for the ground state of H3+ using a basis of floating 1s Gaussian orbitals. The best SCF calculation, thought to be near the Hartree—Fock limit, gives E=−1.29993 a.u. for Re=1.6405 a.u. in equilateral triangular (D3h) geometry. The CI wavefunction which is the best rigorous variational result presently available for H3+ gives E=−1.33764 a.u. at Re=1.6504 a.u. in D3h geometry. It is estimated to lie 0.004–0.01 a.u. above the exact result. The electronic structure of H3+ is interpreted in terms of three-center bonds since the electron density is found to be higher within the triangle than along its sides. Enthalpy, entropy, and Gibbs free energy are computed, and the thermodynamics of several reactions of H3+ examined. In particular, the standard molar enthalpy of formation is found to have a rigorous upper bound of 270 kcal. Variational studies are also presented for D3h H3+ + which give E=−0.12373 a.u. at R=1.68 a.u.