The performance of the M06 family of exchange-correlation potentials for describing the electronic structure and the Heisenberg magnetic coupling constant (J) is investigated using a set of representative open-shell systems involving two unpaired electrons. The set of molecular systems studied has well defined structures, and their magnetic coupling values are known experimentally. As a general trend, the M06 functional is about equally as accurate as B3LYP or PBE0. The performance of local functionals is important because of their economy and convenience for large-scale calculations; we find that M06-L local functional of the M06 family largely improves over the local spin density approximation and the generalized gradient approximation.

The performance of density functional theory in estimating the magnetic coupling constant in a series of Cu(II) binuclear complexes is investigated by making use of two open shell formalisms:  the broken symmetry and the spin-restricted ensemble-referenced Kohn-Sham methods. The strong dependence of the calculated magnetic coupling constants with respect to the exchange-correlation functional is confirmed and found to be independent of whether spin symmetry is imposed or not. The use of a method which guarantees the spin state does not improve the correlation with the experiment and indeed shows some worsening due to an overestimation of the ferromagnetic interactions. However, with the present exchange-correlation functionals, a rather systematic deviation is found. Therefore, it would be possible to develop improved density functionals which will allow for a rigorous treatment of open shell systems in density functional theory.

The dinuclear MnIII complex [Mn2O(PhCOO)2(bpy)2(OH)(NO3)]·H2O was prepared by controlled oxidation of manganous nitrate with n-tetrabutylammonium permanganate in the presence of benzoic acid (PhCOOH) and 2,2‘-bipyridine (bpy). Its structure was determined in a single-crystal X-ray diffraction experiment, and consists of a triply-bridged [Mn2(μ-O)(μ-PhCOO)2]2+ dinuclear core. Each manganese(III) ion bears a chelating bpy and a terminal X anion (X = OH- or NO3 -) completing a distorded octahedral coordination geometry. Although the terminal anions are located in disordered positions, the analysis of bond lengths and steric considerations led us to assign to the complex an asymmetric structure [(bpy)(OH)MnIII(μ-O)(μ-PhCOO)2MnIII(bpy)(ONO2)]. The product, with a chemical formula C34H29Mn2N5O10, crystallizes in the monoclinic system, space group C2/c, with a = 16.607(4) Å, b = 25.619(6) Å, c = 9.796(3) Å, β = 100.15(3)°, and Z = 4. Magnetic studies performed on a series of related compounds have revealed a moderate ferro- or antiferromagnetic interaction and significant zero-field splittings in line with the strong Jahn−Teller distortion of the high spin d4 manganese(III) ions. In the present work we interpreted the magnetic properties of the complex using a spin Hamiltonian which includes the Heisenberg exchange, axial and rhombic ZFS, and an anisotropic Landé factor, under the assumption of a pseudo-C 2 symmetry of the dinuclear core. In order to determine the anisotropy parameters with enough accuracy, we resorted to variable-temperature variable-field magnetization measurements over the 2−300 K range at fields of 0.5, 1.0, 2.5, and 5 T. The whole set of data was fit with a single set of parameters through diagonalization of the complete spin Hamiltonian. The best fit values J = +1.0(4) cm-1, D = +4.5(5) cm-1, E = 0, g x = g y = 1.96, and g z = 2.00 showed that a ferromagnetic interaction occurs between manganese(III) ions in a compressed octahedral environment. A magnetostructural relationship linking the ferromagnetic behavior of the dinuclear complex to the compression of the Mn(III) coordination sphere (and conversely of the antiferromagnetism to the elongation) is then proposed. It is substantiated by theoretical molecular orbital calculations of the extended Hückel type.

Here we present a systematic study on the performance of different GW approaches: GW, GW with linearized quasiparticle equation (lin-GW), and quasiparticle self-consistent GW (qsGW), in predicting core level binding energies (CLBEs) on a series of representative molecules comparing to Kohn-Sham (KS) orbital energy-based results. KS orbital energies obtained using the PBE functional are 20-30 eV lower in energy than experimental values obtained from X-ray photoemission spectroscopy (XPS), showing that any Koopmans-like interpretation of KS core level orbitals fails dramatically. Results from qsGW lead to CLBEs that are closer to experimental values from XPS, yet too large. For the qsGW method, the mean absolute error is about 2 eV, an order of magnitude better than plain KS PBE orbital energies and quite close to predictions from ΔSCF calculations with the same functional, which are accurate within ∼1 eV. Smaller errors of ∼0.6 eV are found for qsGW CLBE shifts, again similar to those obtained using ΔSCF PBE. The computationally more affordable GW approximation leads to results less accurate than qsGW, with an error of ∼9 eV for CLBEs and ∼0.9 eV for their shifts. Interestingly, starting GW from PBE0 reduces this error to ∼4 eV with a slight improvement on the shifts as well (∼0.4 eV). The validity of the GW results is however questionable since only linearized quasiparticle equation results can be obtained. The present results pave the way to estimate CLBEs in periodic systems where ΔSCF calculations are not straightforward although further improvement is clearly needed.

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In the present work we present a comprehensive study of the magneto-structural correlations of a series of ferromagnetic triply heterobridged Cu(II) dinuclear compounds containing [Cu(2)(mu-O(2)CR)(mu-OH)(mu-X)(L)(2)](2+) ions (where X = OH(2), Cl(-), OMe(-) and L = bpy, phen, dpyam) which have the particularity of being all ferromagnetic. The present theoretical study, based on hybrid density functional theory (DFT) calculations, leads to strong conclusions about the role of the pentacoordination geometry of the Cu(II) ions (square base pyramidal (SP) or trigonal bipyramidal (TBP) coordination) and the nature of the third bridging ligand in determining the final value of the magnetic coupling constants in this series of compounds. These investigations point toward the existence of a maximum value for the ferromagnetic interaction and may offer some useful information to synthetic chemists aiming at obtaining new compounds with enhanced ferromagnetism.

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The first ferromagnetic tetranuclear Ni(II) complex with an end-on µ-azido bridge has been synthesized. ThisThe X-ray crystal structure has been solved. The complex Ci2H38N2oNi404[C104]2 crystallizes in the monoclinic system, space group C2/c, with a = 27.238(3) A,¿> = 8.712(1) A, c = 19.208(2) A, ß = 134.10(2)°, V= 3273(1) A* 123, Z = 4, R = 0.030 and J?w = 0.039. The four Ni(II) atoms are in a distorted octahedral environment and are related by an S4 symmetry axis forming a quasi-perfect square. The oxygen atom of the amine ligands (OHpn and Opn) also acts as a bridging ligand between two Ni(II) ions. The magnetic properties of this new complex have been studied. The curve xmTvs T indicates a ferromagnetic coupling between Ni(II) ions. Using the Hamiltonian H = -J{S\S2 + S2S¡ + S3S4 + S4S1), the/value obtained is +21.3 cm-1. Anisotropic perturbation containing both an axial Zeeman effect and ZFS acting within the S = 4 ground state has also been studied.