Double spin-flip approach within equation-of-motion coupled cluster and configuration interaction formalisms: Theory, implementation, and examples

The spin-flip (SF) approach is extended to excitations that flip the spin of two electrons to describe multiconfigurational M-s=0 wave functions via high spin quintet references. Equations and implementation of the double SF (2SF) approach within equation-of-motion coupled-cluster (EOM-CC) and configuration interaction (CI) formalisms are presented. The numerical performance of the resulting EOM-2SF-CC and 2SF-CI models is demonstrated by calculations of symmetric dissociation of O-H bonds in water, electronic states of linear H-4, double CC bond-breaking in ethylene, and low-lying states of trimethylenemethyl diradical and 2,4-didehydrometaxylylene tetraradical. The results of active-space variants of 2SF are very close to the more computationally expensive full-space counterparts. An efficient implementation of the active-space approximation of the 2SF-configuration interaction doubles (CID) model termed 2SF-configuration interaction singles (CIS) is also reported. The scaling of 2SF-CIS is only N-4, which allows applications to relatively large molecules.