Relative energies and geometries of the cis- and trans -HO₃ radicals from the parametric 2-electron density matrix method

The parametric 2-electron reduced density matrix (2-RDM) method employing the M functional [ Mazziotti, D. A.Phys. Rev. Lett. 2008, 101, 253002 ], also known as the 2-RDM(M) method, improves on the accuracy of coupled electron-pair theories including coupled cluster with single-double excitations at the computational cost of configuration interaction with single-double excitations. The cis- and trans-HO₃ isomers along with their isomerization transition state were examined using the recent extension of 2-RDM(M) to nonsinglet open-shell states [ Schwerdtfeger, C. A.; Mazziotti, D. A.J. Chem. Phys. 2012, 137, 034107 ] and several coupled cluster methods. We report the calculated energies, geometries, natural-orbital occupation numbers, and reaction barriers for the HO₃ isomers. We find that the 2-RDM(M) method predicts that the trans isomer of HO₃ is lower in energy than the cis isomer by 1.71 kcal/mol in the correlation-consistent polarized valence quadruple-ζ (cc-pVQZ) basis set and 1.84 kcal/mol in the augmented correlation-consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set. Results include the harmonic zero-point vibrational energies calculated in the correlation-consistent polarized valence double-ζ basis set. On the basis of the results of a geometry optimization in the augmented correlation consistent polarized valence triple-ζ basis set, the parametric 2-RDM(M) method predicts a central oxygen-oxygen bond of 1.6187 Å. We compare these energies and geometries to those predicted by three single-reference coupled cluster methods and experimental results and find that the inclusion of multireference correlation is important to describe properly the relative energies of the cis- and trans-HO₃ isomers and improve agreement with experimental geometries.