Quantum Chemical Characterization of Single Molecule Magnets Based on Uranium.

Multiconfigurational electronic structure theory calculations including spin-orbit coupling effects were performed on four uranium-based single-molecule-magnets. Several quartet and doublet states were computed and the energy gaps between spin-orbit states were then used to determine magnetic susceptibility curves. Trends in experimental magnetic susceptibility curves were well reproduced by the calculations, and key factors affecting performance were identified.

Biradical character in the ground state of [Mn@Si12](+): a DFT and CASPT2 study.

Both density functional theory and multi-configurational ab initio (CASPT2) calculations are used to explore the potential energy surface of the hexagonal prismatic cluster [Mn@Si12](+). Unlike isoelectronic Cr@Si12, the ground state is a biradical, with triplet and open-shell singlet states lying very close in energy. The results are discussed in the context of recent experimental studies using infra-red multiple photon dissociation spectroscopy and X-ray MCD spectroscopy.

Theoretical determination of spin Hamiltonians with isotropic and anisotropic magnetic interactions in transition metal and lanthanide complexes.

The determination of anisotropic magnetic parameters is a task of both experimental and theoretical interest. The added value of theoretical calculations can be crucial for analyzing experimental data by (i) allowing assessment of the validity of the phenomenological spin Hamiltonians, (ii) allowing discussion of the values of parameters extracted from experiments, and (iii) proposing rationalizations and magneto-structural correlations to better understand the relations between geometry, electronic structure, and properties. In this review, we discuss the model Hamiltonians that are used to describe magnetic properties, the computational approaches that can be used to compute magnetic parameters, and review their applications to transition metal and (to a lesser extent) lanthanide based complexes. Perspectives concerning current methodological challenges will then be presented, and finally the need for further joint experimental/theoretical efforts will be underlined.

Rigorous Extraction of the Anisotropic Multispin Hamiltonian in Bimetallic Complexes from the Exact Electronic Hamiltonian.

The magnetic anisotropy of the [Ni2(en)4Cl2](2+) (en = ethylenediamine) complex has been studied using wave function based computational schemes. The spin-orbit state interaction methodology provides accurate ab initio energies and wave functions that are used to interpret the anisotropy in bimetallic complexes. The extraction of the anisotropic spin Hamiltonian is performed using the effective Hamiltonian theory. This procedure which has successfully been applied to mononuclear complexes enables one to solve the weak exchange limit. It is shown that the standard coupled spin Hamiltonian only describes a part of the anisotropy of the molecule. Important higher order terms such as the biquadratic anisotropic exchange should be included in the model for an appropriate description of the anisotropy.

Universal Theoretical Approach to Extract Anisotropic Spin Hamiltonians.

Monometallic Ni(II) and Co(II) complexes with large magnetic anisotropy are studied using correlated wave function based ab initio calculations. Based on the effective Hamiltonian theory, we propose a scheme to extract both the parameters of the zero-field splitting (ZFS) tensor and the magnetic anisotropy axes. Contrarily to the usual theoretical procedure of extraction, the method presented here determines the sign and the magnitude of the ZFS parameters in any circumstances. While the energy levels provide enough information to extract the ZFS parameters in Ni(II) complexes, additional information contained in the wave functions must be used to extract the ZFS parameters of Co(II) complexes. The effective Hamiltonian procedure also enables us to confirm the validity of the standard model Hamiltonian to produce the magnetic anisotropy of monometallic complexes. The calculated ZFS parameters are in good agreement with high-field, high-frequency electron paramagnetic resonance spectroscopy and frequency domain magnetic resonance spectroscopy data. A methodological analysis of the results shows that the ligand-to-metal charge transfer configurations must be introduced in the reference space to obtain quantitative agreement with the experimental estimates of the ZFS parameters.