Why spin contamination is a major problem in the calculation of spin–spin coupling in triplet biradicals

In organic triplet biradicals, zero field splitting is dominated by the spin–dipolar interaction. While it has been observed before that spin-unrestricted density functional calculations give huge errors for systems with small D values, a consistent explanation was still missing. Using model systems, this work demonstrates how spin contamination creates unphysical one-center terms in the two-particle density matrix that spoil the calculated results.