Coupled-cluster studies of HOONO⁺: Additional conformers of the ²A″ON⁺OOH complex and a comparison of restricted and unrestricted open-shell Hartree–Fock coupled-cluster results

Coupled-cluster calculations using the CCSD(T) method (coupled-cluster singles and doubles with perturbative inclusion of triple excitations) and correlation-consistent basis sets have been used to address aspects of the lowest ²A′ and ²A″ potential energy surfaces of the peroxynitrous acid cation (HOONO⁺). First, searches were made for conformers of the ²A″ ON⁺OOH complex in addition to the cis–trans species previously located by the authors (J Chem Phys 2003, 118, 1721). Two conformers were located—namely, trans–cis and trans–trans species—and these were confirmed to be local minima. The trans–trans conformer was calculated to be less than 0.5 kcal mol⁻¹ higher in energy than the cis–trans conformer, and the trans–cis conformer was calculated to be about 6.4 kcal mol⁻¹ higher in energy than the cis–trans conformer. Second, comparisons were made between CCSD(T) results obtained with unrestricted Hartree–Fock (UHF) and restricted open-shell Hartree–Fock (ROHF) reference determinants. These comparisons were made on the ²A″ complexes and the planar ²A′ HOONO⁺ stationary points. In general, the energy differences between UHF-CCSD(T) and ROHF-CCSD(T) were very small, and well within the error bars of the methods. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004