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Determining the energy gap between the cis and trans isomers of HO3- using geometry optimization within the anti-Hermitian contracted Schrödinger and coupled cluster methods.

The cis and trans isomers of the HO3- anion, which are important in proposed mechanisms for ozonization, are studied computationally. Relative energies, geometries, and normal-mode frequencies are calculated with anti-Hermitian contracted Schrödinger equation (ACSE) and coupled cluster methods. Both the ACSE method and the coupled cluster method with single and double excitations (CCSD) are applied in a correlation-consistent polarized double-zeta basis set (cc-pVDZ). Using coupled cluster with singles, doubles, and perturbative triples (CCSD(T)), we treat the problem with larger basis sets than those in previous work, including correlation-consistent polarized quadruple-zeta basis sets with (aug-cc-pVQZ) and without (cc-pVQZ) diffuse functions, which permit extrapolation of the cis and trans energies to the complete-basis-set limit. The cis isomer is found to be lower in energy than the trans isomer by -3.5 kcal/mol, which is 50% larger in magnitude than the best previous result of -2.2 kcal/mol. The bond lengths between the O2 and OH fragments of the cis- and trans-HO3 are calculated to be 1.713 and 1.857 A, respectively, where both bond lengths are significantly longer than the 1.464 A O-O bond in hydrogen peroxide. In this paper, we extend the ACSE method [Mazziotti, D. A. J. Chem. Phys. 2007, 126, 184101], which computes the two-electron reduced density matrix directly, to include geometry optimization by a Newton's method with numerical derivatives. Calculation of the cis- and trans-HO3- isomers by the ACSE yields energies, geometries, and frequencies that are closer to those from CCSD(T) than those from CCSD.