The protonated water dimer: Brueckner methods remove the spurious C1 symmetry minimum

Abstract

The H5O2+ system has been studied using a variety of coupled cluster methods based on a Brueckner reference determinant with levels of correlation up to double and perturbatively treated connected triple excitations [B–CCD(T)]. Basis sets as large as the triple-ζ plus double polarization basis augmented with f functions on oxygen and d functions on hydrogen [TZ2P(f,d)] were used. Harmonic vibrational frequencies were also predicted. In contrast with previous high-level ab initio studies, a stationary point of C1 symmetry was not found. An absence of imaginary vibrational frequencies at all levels of theory for the stationary point of C2 symmetry proves it to be the global minimum, lying only ∼0.4 kcal/mol lower in energy than the transition state of Cs symmetry.