Spin-flip methods in quantum chemistry
This Perspective discusses salient features of the spin-flip approach to strong correlation and describes different methods that sprung out of this idea. The spin-flip treatment exploits different physics of low-spin and high-spin states and is based on the observation that correlation is small for the same-spin electrons. By using a well behaved high-spin state as a reference, one can access problematic low-spin states by deploying the same formal tools as in the excited-state treatments (i.e., linear response, propagator, or equation-of-motion theories). The Perspective reviews applications of this strategy within wave-function and density functional theory frameworks as well as the extensions for molecular properties and spectroscopy. The utility of the spin-flip methods is illustrated by examples. The limitations and proposed future directions are also discussed.