title: Reinvestigation of the reaction of ethylene and singlet oxygen by the approximate spin projection method. Comparison with multireference coupled-cluster calculations.
abstract: We quantify a spin contamination error caused by a broken-symmetry (BS) method on the geometry at the stationary points and barrier heights of the [2 + 2] reaction between singlet oxygen and ethylene, which goes through a diradical intermediate. Several hybrid GGA, hybrid meta-GGA, and long-range corrected hybrid functionals, O3LYP, B3LYP, PBE0, MPW1B95, BHandHLYP, and omegaB97X, are examined to elucidate their original nature without the spin contamination error. For that purpose, the geometry of each reaction step for the BS state as well as its total energy is corrected by using an approximate spin projection method. The CCSD and CCSD(T) single-point calculations are also carried out at optimized geometries at the DFT level to confirm the results of the DFT methods. The single-point calculations by means of Mukherjee's multireference coupled cluster with single and double excitations at CASSCF(10e,8o)-optimized geometries are also presented to assess the DFT methods. After the energy and geometry corrections, the barrier height of each functional is consistent with conventional closed-shell-type reactions even in the reaction involving singlet diradical species. We also find that the spin contamination error on the geometric change is not negligible especially at the early stage of the reaction ( approximately 3 kcal/mol), where the triplet state is the ground state.
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. tsaito@chem.sci.osaka-u.ac.jp
authors.forename: Toru
authors.initials: T
authors.lastname: Saito
authors.forename: Satomichi
authors.initials: S
authors.lastname: Nishihara
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Kataoka
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Nakanishi
authors.forename: Yasutaka
authors.initials: Y
authors.lastname: Kitagawa
authors.forename: Takashi
authors.initials: T
authors.lastname: Kawakami
authors.forename: Shusuke
authors.initials: S
authors.lastname: Yamanaka
authors.forename: Mitsutaka
authors.initials: M
authors.lastname: Okumura
authors.forename: Kizashi
authors.initials: K
authors.lastname: Yamaguchi