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 augme...