The H+5 potential energy hypersurface: Characterization of ten distinct energetically low-lying stationary points

Abstract

Ab initio molecular electronic structure theory has been used in an attempt to characterize the low‐lying stationary points on the H⁺₅ potential energy hypersurface. Three distinct levels of theory have been used: the self‐consistent‐field (SCF) method, configuration interaction (CI) including all single and double excitations, and full configuration interaction. Four different basis sets were used: double zeta (DZ), double zeta plus polarization (DZP), an extended basis set designated H (6s2p/4s2p), and a second extended basis set designated H (8s3p/6s3p). The higher levels of theory are in agreement that the only minimum for H⁺₅ is a C_2v structure, with three other stationary points (of D_2d, C_2v, and D_2h symmetries) lying less than 1 kcal/mol higher in energy. The predicted dissociation energy D₀ is 5.5 kcal/mol, which is estimated to be about 1 kcal/mol less than the exact D₀. Furthermore, there are six other stationary points lying less than 8 kcal/mol above the minimum. Vibrational frequencies, dipole moments, and infrared intensities for each of the ten stationary points have been predicted at several different levels of theory. From the perspective of quantum chemistry, the H⁺₅ system is very attractive as a candidate for the study of the vibrational dynamics of weakly bound systems.