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7.10 Coupled-Cluster Excited-State and Open-Shell Methods

7.10.5 Spin-Flip Methods for Di- and Triradicals

(December 20, 2021)

The spin-flip method 600 Krylov A. I.
Chem. Phys. Lett.
(2001), 338, pp. 375.
, 601 Krylov A. I.
Chem. Phys. Lett.
(2002), 350, pp. 522.
, 602 Krylov A. I.
Acc. Chem. Res.
(2006), 39, pp. 83.
addresses the bond-breaking problem associated with a single-determinant description of the wave function. Both closed and open shell singlet states are described within a single reference as spin-flipping, (e.g., αβ excitations from the triplet reference state), for which both dynamical and non-dynamical correlation effects are smaller than for the corresponding singlet state. This is because the exchange hole, which arises from the Pauli exclusion between same-spin electrons, partially compensates for the poor description of the coulomb hole by the mean-field Hartree-Fock model. Furthermore, because two α electrons cannot form a bond, no bond breaking occurs as the internuclear distance is stretched, and the triplet wave function remains essentially single-reference in character. The spin-flip approach has also proved useful in the description of di- and tri-radicals as well as some problematic doublet states.

The spin-flip method is available for the CIS, CIS(D), CISD, CISDT, OD, CCSD, and EOM-(2,3) levels of theory and the spin complete SF-XCIS (see Section 7.2.5). An N7 non-iterative triples corrections are also available. For the OD and CCSD models, the following non-relaxed properties are also available: dipoles, transition dipoles, eigenvalues of the spin-squared operator (S2), and densities. Analytic gradients are also for SF-CIS and EOM-SF-CCSD methods. Construction of effective Hamiltonians in Heisenberg and Hubbard spaces from EOM-SF wave functions is described in the Section 13.6. To invoke a spin-flip calculation the SF_STATES $rem should be used, along with the associated $rem settings for the chosen level of correlation by using METHOD (recommended) or using older keywords (CORRELATION, and, optionally, EOM_CORR). Note that the high multiplicity triplet or quartet reference states should be used.

Several double SF methods have also been implemented. 167 Casanova D. et al.
J. Chem. Phys.
(2009), 130, pp. 044103.
To invoke these methods, use DSF_STATES.