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

Abstract

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.