Ab initio potential energy and dipole moment surfaces for H5O2+

Abstract

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H5O2+. Tens of thousands of coupled-cluster [CCSD(T)] and second-order Møller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H5O2+ PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm−1 for the entire data set up to 110 000 cm−1. This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H2O+H3O+, with De=11 923.8 cm−1. Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.