The parametric 2-electron reduced density matrix (2-RPM) 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-zeta (cc-pVQZ) basis set and 1.84 kcal/mol in the augmented correlation consistent polarized valence quadruple-zeta (aug-cc-pVQZ) basis set Results include the harmonic zero point vibrational energies calculated in the correlation consistent polarized valence double-zeta basis set On the basis of the results of a geometry optimization in the augmented correlation consistent polarized valence triple-zeta basis set, the parametric 2-RDM(M) method predicts a central oxygen-oxygen bond of 1.6187 angstrom. 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.