title: Potential energy curve for ring-opening reactions: comparison between broken-symmetry and multireference coupled cluster methods.
abstract: The spin-unrestricted Hartree-Fock (UHF)-based coupled cluster singles and doubles (UHF-CCSD) and Mukherjee's state-specific multireference CCSD (MkCCSD) methods are applied to four ring-opening reactions. The spin-restricted Hartree-Fock (RHF)-based CCSD (RHF-CCSD) calculations are also performed for comparison. In the case of the UHF-CCSD method, an approximate spin-projection (AP) method is applied to the broken-symmetry (BS) singlet solution to remove the spin contamination effect. For potential energy curves (PECs) of all reactions presented in this study, the results of RHF-CCSD and UHF-CCSD are substantially different from those of MkCCSD, while the results after the AP method (AP-UCCSD) reproduce the MkCCSD results well. It strongly suggests that the spin contamination effect should be removed by the AP correction even at the UHF-CCSD level to predict reliable energetics of these reactions.
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: Natsumi
authors.initials: N
authors.lastname: Yasuda
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