Double spin-flip approach within equation-of-motion coupled cluster and conguration interaction formalisms: Theory, implementation and examples
The spin-flip (SF) approach is extended to Ms=-1 excitations that flip the spin of two electrons to describe multi-congurational Ms=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 conguration 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 H4, 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-CID model termed 2SF-CIS is also reported. The scaling of 2SF-CIS is only N4, which allows applications to relatively large molecules.