The Perdew–Zunger (PZ) self-interaction correction
(SIC)
is an established tool to correct unphysical behavior in density functional
approximations. Yet, the PZ-SIC is well-known to sometimes break molecular
symmetries. An example of this is the benzene molecule, for which
the PZ-SIC predicts a symmetry-broken electron density and molecular
geometry, since the method does not describe the two possible Kekulé
structures on an even footing, leading to local minima [J. Chem. Theory Comput.2016123195]. The PZ-SIC is often implemented with Fermi–Löwdin
orbitals (FLOs), yielding the FLO-SIC method, which likewise has issues
with symmetry breaking and local minima [J. Chem. Phys.2021155224109]. In this work, we propose a generalization of the PZ-SICthe
ensemble PZ-SIC (E-PZ-SIC) methodwhich shares the asymptotic
computational scaling of the PZ-SIC (albeit with an additional prefactor).
The E-PZ-SIC is straightforwardly applicable to various molecules,
merely requiring one to average the self-interaction correction over
all possible Kekulé structures, in line with chemical intuition.
We showcase the implementation of the E-PZ-SIC with FLOs, as the resulting
E-FLO-SIC method is easy to realize on top of an existing implementation
of the FLO-SIC. We show that the E-FLO-SIC indeed eliminates symmetry
breaking, reproducing a symmetric electron density and molecular geometry
for benzene. The ensemble approach suggested herein could also be
employed within approximate or locally scaled variants of the PZ-SIC
and its FLO-SIC versions.