title: Spin contamination error in optimized geometry of singlet carbene ((1)A1) by broken-symmetry method.
abstract: Spin contamination errors of a broken-symmetry (BS) method in optimized structural parameters of the singlet methylene ((1)A(1)) molecule are quantitatively estimated for the Hartree-Fock (HF) method, post-HF methods (CID, CCD, MP2, MP3, MP4(SDQ)), and a hybrid DFT (B3LYP) method. For the purpose, the optimized geometry by the BS method is compared with that of an approximate spin projection (AP) method. The difference between the BS and the AP methods is about 10-20 degrees in the HCH angle. In order to examine the basis set dependency of the spin contamination error, calculated results by STO-3G, 6-31G*, and 6-311++G** are compared. The error depends on the basis sets, but the tendencies of each method are classified into two types. Calculated energy splitting values between the triplet and the singlet states (ST gap) indicate that the contamination of the stable triplet state makes the BS singlet solution stable and the ST gap becomes small. The energy order of the spin contamination error in the ST gap is estimated to be 10(-1) eV.
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. kitagawa@chem.sci.osaka-u.ac.jp
authors.forename: Yasutaka
authors.initials: Y
authors.lastname: Kitagawa
authors.forename: Toru
authors.initials: T
authors.lastname: Saito
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Nakanishi
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Kataoka
authors.forename: Toru
authors.initials: T
authors.lastname: Matsui
authors.forename: Takashi
authors.initials: T
authors.lastname: Kawakami
authors.forename: Mitsutaka
authors.initials: M
authors.lastname: Okumura
authors.forename: Kizashi
authors.initials: K
authors.lastname: Yamaguchi