Charge density analysis of triplet and broken symmetry states relevant to magnetic coupling in systems with localized spin moments

Comparison of electron charge density for triplet and broken symmetry solut ions obtained from different computational methods together with the atoms- in-molecules (AIM) and electron localization function (ELF) topological ana lyses support the description of these systems through the Heisenberg Hamil tonian. The reduction of the low-energy spectrum to a purely spin Hamiltoni an holds for all studied methods, although local density approximation (LDA ) exhibits some noticeable deviation. The analysis of charge difference den sity plots clearly shows that the failure of LDA to describe magnetic coupl ing is due to the too-strong delocalization that leads to a qualitatively i ncorrect electron density in the region near the nuclei. Gradient-corrected and hybrid functionals correct this defect but in an exaggerated way. The role of mixing Fock exchange is also discussed.