Claude Daul

DFT calculations of molecular magnetic properties of coordination compounds

Abstract An overview of density functional theory (DFT) based techniques for the calculation of the magnetic properties of molecular and supramolecular assemblies is presented. Three different approaches to compute the exchange coupling constant (Jex) are reviewed, i.e. the broken symmetry (BS) technique, the single determinant (SD) approach, and the spin projection method. The first one (BS), developed by Noodleman, is undoubtedly most commonly applied, e.g. to clusters containing several paramagnetic metal centres or to paramagnetic organic radical species. The second approach (SD) was originally developed to compute the electronic spectra of transition metal complexes, but was more recently applied to the computation of spin manifold of molecular magnets. The last method, developed by Ovchinnikov and Labanowsky, is mainly an extension of the Hartree–Fock (HF) concept of spin de-contamination to DFT. The performance of the three methods has been evaluated for model systems (HHeH, [Cu2Cl6]2−) and for more complex molecules (Ti(CatNSQ)2 and Sn(CatNSQ)2, Bis-verdazyl diradical (BVD), [Fe2(OH)3(tmtacn)2]2+ and [[Cu3O2L3]3+, L=N-Permethylated (1R,2R)-cyclohexanediamine). A comparison of these results with experimental values and with post-HF results if available is presented as well. In the case of the last two complexes, i.e. mixed valence systems, computation of the vibronic potential energy surfaces is also briefly discussed.

New insights into the effects of covalency on the ligand field parameters: a DFT study

Abstract A new, non-empirical, DFT based ligand field (LF) model is proposed. The calculation involves two steps: (i) an average of configuration (AOC), with equal occupation of the d-orbitals is carried out, (ii) with these Kohn–Sham orbitals kept frozen, the energies of all single determinants (SD) within the whole LF-manyfold is performed. These energies are then used to estimate all the Racah- and LF-parameters needed in a conventional LF-calculation. The results of this first-principle prediction are in very good agreement with the experimental values. Test calculation of tetrahedral Cr (IV) and Ni (II) complexes are used to validate the new model and to analyze the parameters of the LF.

Combined Ligand Field and Density Functional Theory Analysis of the Magnetic Anisotropy in Oligonuclear Complexes Based on FeIII-CN-MII Exchange-Coupled Pairs

Magnetic anisotropy in cyanide-bridged single-molecule magnets (SMMs) with Fe(III)-CN-M(II) (M = Cu, Ni) exchange-coupled pairs was analyzed using a density functional theory (DFT)-based ligand field model. A pronounced magnetic anisotropy due to exchange was found for linear Fe(III)-CN-M(II) units with fourfold symmetry. This results from spin-orbit coupling of the [Fe(III)(CN)6](3-) unit and was found to be enhanced by a tetragonal field, leading to a (2)E g ground state for Fe(III). In contrast, a trigonal field (e.g., due to tau 2g Jahn-Teller angular distortions) led to a reduction of the magnetic anisotropy. A large enhancement of the anisotropy was found for the Fe(III)-CN-Ni(II) exchange pair if anisotropic exchange combined with a negative zero-field splitting energy of the S = 1 ground state of Ni(II) in tetragonally compressed octahedra, while cancellation of the two anisotropic contributions was predicted for tetragonal elongations. A recently developed DFT approach to Jahn-Teller activity in low-spin hexacyanometalates was used to address the influence of dynamic Jahn-Teller coupling on the magnetic anisotropy. Spin Hamiltonian parameters derived for linear Fe-M subunits were combined using a vector-coupling scheme to yield the spin Hamiltonian for the entire spin cluster. The magnetic properties of published oligonuclear transition-metal complexes with ferromagnetic ground states are discussed qualitatively, and predictive concepts for a systematic search of cyanide-based SMM materials are presented.

Full-CI quantum chemistry using the density matrix renormalization group

We describe how density matrix renormalization group (DMRG) can be used to solve the full configuration interaction problem in quantum chemistry. As an illustration of the potential of this method, we apply it to a paramagnetic molecule. In particular, we show the effect of various basis set, the scaling as the fourth power of the size of the problem, and compare the DMRG with other methods.

Average Voltage, Energy Density, and Specific Energy of Lithium‐Ion Batteries Calculation Based on First Principles

The theoretical average voltage, energy density (energy per volume), and specific energy (energy per mass) based on the active electrode material have been calculated from first principles for two types of rechargeable lithium-ion batteries. In the charged state the two batteries consist of LiC{sub 6} and Mo{sub 2} electrodes (M = Mo and Ni). The calculation was performed using the linearized augmented plane wave crystal code WIEN95 based on density functional theory (DFT). The structure was calculated by varying the unit cell volume of the experimentally known crystallographic data with respect to the total energy. The calculated results are compared with measured values. The temperature dependence of the average voltage, energy density, and specific energy was demonstrated to be of minor importance. In the case of the LiC{sub 6}/NiO{sub 2} battery this was done by calculating the vibrational energy contribution to the enthalpy change using the cluster approximation and the Amsterdam density functional (ADF) molecular code based on DFT. The agreement between theoretical and experimental values opens up the use of first principles quantum chemistry in battery technology.

Experimental study of the second-order non-linear optical properties of tetrathia-[7]-helicene

Abstract Femtosecond hyper-Rayleigh scattering (HRS) experiments as a function of amplitude modulation frequency have been performed on tetrathia-[7]-helicene. The apparent first hyperpolarizability β app ( ω ) is a decreasing function of increasing frequency ω , due to the two-photon fluorescence (2PF) contribution. We observe, however, for the first time, also a frequency-dependent HRS depolarization ratio ρ ( ω ). An accurate value for ρ is important, since it offers an indication for the symmetry of the chromophore (5, resp. 1.5, for dipoles, resp. octopoles, but also independent of frequency, since identical for HRS and 2PF). For helicoidal symmetry ρ ( ω ) is increasing for higher frequencies, in agreement with theory.

Correct dissociation behavior of radical ions such as H2+ in density functional calculations

In this contribution it is shown that the unphysical dissociation energy curves of dimeric ions bearing a small odd number of electrons as obtained with DFT calculations can be cured by a posteriori corrections. The self-interaction error, which is known to be at the origin of the unphysical dissociation behavior, is corrected by a Slater’s transition state calculation. A very satisfactory dissociation energy curve is obtained for He2+. However for H2+, it is also necessary to introduce fractional occupation numbers to obtain a good description of the system.

Excited-state energies and distortions of d0 transition metal tetraoxo complexes: A density functional study

Excitation energies and excited-state distortions of 9 tetrahedral transition metal tetraoxo complexes with a formal d0 electron configuration have been investigated using density functional theory. A symmetry based calculation scheme was applied for the 3T2, 3T1, 1T2, and 1T1 states deriving from the first excited electron configurations. The multiplet method was combined with a transition state approach for the calculation of the excitation energies. The results are compared with those from experiments, and with other calculations. The experimental ground-state properties are very well reproduced. The calculated absorption energies are slightly overestimated, but with an overall very good agreement. Potential-energy curves are calculated for both the ground and first excited states. The experimentally determined expansion of the excited state as well as the reduction in the vibrational frequencies are reproduced by the calculation. The bonding in this series of complexes is characterized by their strong...

A density functional investigation of the ground‐ and excited‐state properties of ruthenocene

Quantum chemical calculations based on density functional theory have been performed on ruthenocene. Excellent agreement is obtained with ground‐ and excited‐state properties derived from optical spectroscopy. In particular, the energies of the first d–d excitations, the unusually large Stokes shift, the structural expansion of Ru(cp)2 and the substantial reduction of the Ru‐cp force constant in the first triplet excited state are almost quantitatively reproduced. The lowest‐energy excitation is found to have substantial charge transfer character.

Ligand field density functional theory calculation of the 4f2 → 4f15d1 transitions in the quantum cutter Cs2KYF6:Pr3+

Herein we present a Ligand Field Density Functional Theory (LFDFT) based methodology for the analysis of the 4f(n)→ 4f(n-1)5d(1) transitions in rare earth compounds and apply it for the characterization of the 4f(2)→ 4f(1)5d(1) transitions in the quantum cutter Cs2KYF6:Pr(3+) with the elpasolite structure type. The methodological advances are relevant for the analysis and prospection of materials acting as phosphors in light-emitting diodes. The positions of the zero-phonon energy corresponding to the states of the electron configurations 4f(2) and 4f(1)5d(1) are calculated, where the praseodymium ion may occupy either the Cs(+)-, K(+)- or the Y(3+)-site, and are compared with available experimental data. The theoretical results show that the occupation of the three undistorted sites allows a quantum-cutting process. However size effects due to the difference between the ionic radii of Pr(3+) and K(+) as well as Cs(+) lead to the distortion of the K(+)- and the Cs(+)-site, which finally exclude these sites for quantum-cutting. A detailed discussion about the origin of this distortion is also described.