In June of 2022, the FLOSIC team published an article in the Journal of Chemical Physics that introduced the use of complex Fermi orbital descriptors (FODs) in the Fermi–Löwdin self-interaction-corrected density functional theory (FLOSIC).
Complex FLO-SIC (cFLOSIC) calculations based on the local spin density approximation produce total energies that are generally lower than the corresponding energies found with FLOSIC restricted to real orbitals (rFLOSIC). The cFLOSIC results are qualitatively similar to earlier Perdew–Zunger SIC (PZ-SIC) calculations using complex orbitals [J. Chem. Phys. 80, 1972 (1984); Phys. Rev. A 84, 050501(R) (2011); and J. Chem. Phys. 137, 124102 (2012)].
The energy lowering stems from the exchange–correlation part of the self-interaction correction. The Hartree part of the correction is more negative in rFLOSIC. The energy difference between real and complex solutions is greater for more strongly hybridized FLOs in atoms and for FLOs corresponding to double and triple bonds in molecules. The case of N2 is examined in detail to show the differences between the real and complex FLOs.
This publication showed that complex triple-bond orbitals are simple, and discussed physically appealing combinations of π and σg orbitals that had not been discussed before. Consideration of complex FODs, and resulting unitary transformations, underscored the fact that FLO centroids are not necessarily good guesses for FOD positions in a FLOSIC calculation.