Finite-Field Spin-Flip Configuration Interaction Calculation of the Second Hyperpolarizabilities of Singlet Diradical Systems

摘要

Ab initio spin-flip configuration interaction(SF-Cl)methods with the finite-field(FF)scheme are applied to the calculation of static second hyperpolarizabilities(gamma)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(y)dependence of y in a wide range of a diradical character region for H2 under dissociation and 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 y.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-Cl method starting with a pure or a nearly pure high-spin state to provide qualitative or semiquantitative y 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 y on y in the case of p-quinodimethane.