A Combined Spin-Flip and IP/EA Approach for Handling Spin and Spatial Degeneracies: Application to Double Exchange Systems

Many
multiconfigurational systems, such as single-molecule magnets,
are difficult to study using traditional computational methods due
to the simultaneous existence of both spin and spatial degeneracies.
In this work, a new approach termed n-spin-flip ionization
potential/electron affinity (nSF-IP or nSF-EA) is introduced which combines the spin-flip method of Anna
Krylov with particle-number changing IP/EA methods. We demonstrate
the efficacy of the approach by applying it to the strongly correlated
N2+, as well as several double exchange systems.
We also demonstrate that when these systems are well-described by
a double exchange model Hamiltonian, only 1SF-IP/EA is required to
extract the double exchange parameters and accurately predict energies
for the low-spin states. This significantly reduces the computational
effort for studying such systems. The effects of including additional
excitations (using a RAS-nSF-IP/EA scheme) are also
examined, with particular emphasis on hole and particle excitations.