Spin-Density Functional Regularization for Singlet Diradicals

Broken-symmetry density functional theory (DFT) is one of the commonly used methods for treating the static-correlation effects in singlet diradicals, which arise from the (quasi-)degeneracy of their frontier orbitals. Although the method yields quantitatively accurate energies for simple cases, such as stretched diatomic molecules, it fails for other systems with more complicated electronic structures due to the artificially broken spin symmetries. In this work, we present a spin-density functional regularization (SR) approach within the framework of partition density functional theory (PDFT) to correct errors arising from artificial symmetry breaking in broken-symmetry DFT calculations. To demonstrate the efficacy of our method, we examine the automerization of cyclobutadiene, a classic example where the transition state is a singlet diradical. While conventional broken-symmetry DFT calculations using standard exchange-correlation density functional approximations systematically underestimate the energy of the automerization transition state, SR-PDFT effectively eliminates the influences of artificial symmetry breaking and yields chemically accurate barrier heights for the reaction.