Finite-field spin-flip configuration interaction calculation of the second hyperpolarizabilities of singlet diradical systems

Ryohei Kishi, Masayoshi Nakano, Suguru Ohta, Akihito Takebe, Masahito Nate, Hideaki Takahashi, Takashi Kubo, Kenji Kamada, Koji Ohta, Benoît Champagne, Edith Botek

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41 Citations (Scopus)


Ab initio spin-flip configuration interaction (SF-CI) methods with the finite-field (FF) scheme are applied to the calculation of static second hyperpolarizabilities (γ) of several singlet diradical systems, i.e., the model H2 molecule under dissociation, p-quinodimethane, o-quinoid five-membered ring, and 1,4-bis(imidazole-2-ylidene)cyclohexa-2,5-diene (BI2Y) models. The SF-CI method using the UHF reference wave function provides the qualitatively correct diradical character (γ) dependence of γ in a wide range of a diradical character region for H2 under dissociation ahd p-quinodimethane as well as o-quinoid five-membered ring models. For BI2Y, which is a real diradical system, a non-negligible spin contamination is found in the spin-unrestricted Hartree-Fock (UHF) triplet state, which results in overestimations (SF-CIS) or underestimations (SF-CIS(D)) of γ. Such deficiencies are significantly reduced when using the pure spin state, i.e., the restricted open-shell HF (ROHF) triplet wave function as the reference wave function. These results indicate the applicability of the FF-SF-CI method starting with a pure or a nearly pure high-spin state to provide qualitative or semiquantitative γ for large-size diradical systems. For selected systems, these SF-CI results are also compared to the SF equation of motion coupled cluster singles and doubles (SF-EOM-CCSD) and to SF time-dependent density functional theory (SF-TDDFT) schemes. In particular, large amounts of Hartree-Fock exchange in the functional are required to obtain qualitatively correct dependence of γ on y in the case of p-quinodimethane.

Original languageEnglish
Pages (from-to)1699-1707
Number of pages9
JournalJournal of Chemical Theory and Computation
Issue number5
Publication statusPublished - 2007 Sept


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