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The spin-flip method^{509, 510, 511}
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.*, $\alpha \to \beta $ 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 $\alpha $ 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 7.2.4). An N${}^{7}$ 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 ($\u27e8{S}^{2}\u27e9$), 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 13.5.
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.^{132}
To invoke these methods, use DSF_STATES.