Ria Broer

Comment on "about the calculation of exchange coupling constants using density-functional theory: The role of the self-interaction error" [J. Chem. Phys. 123, 164110 (2005)]

The use of density functional theory to obtain energy differences related to magnetic coupling constants is debated with special emphasis to the claims by Ruiz et al. [J. Chem. Phys.123, 164110 (2005)] that good agreement with experiment using the B3LYP potential is obtained by ignoring spin symmetry in case self-interaction error cannot be removed.

Explorative computational study of the singlet fission process

Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the 1TT state can be identified using multi-reference techniques. The geometrical and vibrational properties of the 1TT state are such that they can be approximated with those of the 5TT state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the 1TT and S1 states are close in energy with a large non-adiabatic coupling matrix element at the S1 geometry, and the energy of the S0 state is well below that of the 1TT state at the 1TT geometry. The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The 1TT states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and 1TT[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and 1TT[2] excited states.

Ferromagnetism and increased ionicity in epitaxially grown TbMnO3 films

Thin films of TbMnO3 have been grown on SrTiO3 substrates. The films grow under compressive strain and are only partially clamped to the substrate. This produces remarkable changes in the magnetic properties and, unlike the bulk material, the films display ferromagnetic interactions below the ordering temperature of similar to 40 K. X-ray photoemission measurements in the films show that the Mn 3s splitting is 0.3 eV larger than that of the bulk. Ab initio embedded-cluster calculations yield Mn 3s splittings that are in agreement with the experiment and reveal that the larger observed values are due to a larger ionicity of the films.

Electron correlation effects on the d-d excitations in NiO

The partly filled 3d shell in solid transition metal compounds is quite localized on the transition metal ion and gives rise to large electron correlation effects. With the recently developed CASSCF/CASPT2 approach electron correlation effects can be accounted for efficiently. The CASSCF step accounts for the non-dynamical correlation and part of the dynamical correlation, the following CASPT2 step takes largely care of the remaining dynamical correlation in a perturbative way. This approach is applied to the d-d excitations in NiO for which both non-dynamical and dynamical electron correlation effects have substantial influence on the energy differences. Excitation energies that compare well to the experimental data are obtained and the importance of the different electron correlation effects can be assessed.

Ab initio study of the singlet—triplet splitting in simple models for dichloro- and difluoro-bridged Cu(II) dimers

Abstract The results are reported of an ab initio all-electron self-consistent field (SCF) plus configuration interaction (CI) study of some simple models of dichloro- and difluoro-bridged Cu(II) dimers. The splitting between the lowest singlet and triplet energy levels is found to be strongly dependent on the angle φ (the CuLCu angle) between the copper ions and the bridging ligands L. The singlet—triplet splitting E S  E T shows a maximum for φ between 90° and 100°. For angles in this region the tripl state is lowest, whereas for larger and smaller angles the singlet state is lowest. The strong dependence of the splitting originates predominantly from first-order copper to copper charge transfer contributions to the singlet wavefunction.

Antisymmetric Magnetic Interactions in Oxo-Bridged Copper(II) Bimetallic Systems

The antisymmetric magnetic interaction is studied using correlated wave-function-based calculations in oxo-bridged copper bimetallic complexes. All of the anisotropic multispin Hamiltonian parameters are extracted using spin-orbit state interaction and effective Hamiltonian theory. It is shown that the methodology is accurate enough to calculate the antisymmetric terms, while the small symmetric anisotropic interactions require more sophisticated calculations. The origin of the antisymmetric anisotropy is analyzed, and the effect of geometrical deformations is addressed.

Heisenberg exchange enhancement by orbital relaxation in cuprate compounds

Abstract We calculate the Heisenberg exchange J in the quasi-2D antiferromagnetic cuprates La 2 CuO 4 , YBa 2 Cu 3 O 6 , Nd 2 CuO 4 and Sr 2 CuO 2 Cl 2 . We apply all-electron (MC)SCF and non-orthogonal CI calculations to [Cu 2 O 11 ] 18− , [Cu 2 O 9 ] 14− , [Cu 2 O 7 ] 10− and [Cu 2 O 7 Cl 4 ] 14− cluster in a model charge embedding. The (MC)SCF triplet and singlet ground states are well characterized by Cu 2+ (d x 2 − y 2 ) and O 2− . The antiferromagnetic exchange is strongly enhanced by admixing relaxed (MC)SCF triplet and singlet excited states, in which a single electron is transferred from the central O ion to Cu. We ascribe this effect to orbital relaxation in the charge transfer component of the wavefunction. Close agreement with experiment is obtained.

Broken orbital-symmetry and the description of hole states in the tetrahedral [CrO4]− anion. I. Introductory considerations and calculations on oxygen 1s hole states

Abstract The localization of holes in systems containing spatially equivalent sites is discussed in terms of a simple one-particle model in which quantum mechanical delocalization effects compete with essentially classical polarization or dielectric relaxation effects. The predictions of the model for a tetrahedral system like CrO − 4 compare favourably with the results of symmetry unrestricted SCF calculations on O 1s hole states. The connection with a Cl treatment using symmetry-restricted MOs is discussed. The calculated ionization energies are finally compared with XPS measurements on Na 2 CrO 4 . To this end the crystal surrounding of the CrO − 4 anion has been represented by a point charge model and the ensuing Modelung field was included in the SCF calculations. In contrast to the Td restricted result of 551.4 eV, the completely localized C 3v results of 532.6 eV is in satisfactory agreement with the experimental data which are found around 530.0 eV.

Broken orbital symmetry and the description of valence hole states in the tetrahedral [CrO4]2- anion

The localization of ligand-based valence holes in the tetrahedral complex ion [CrO4]2− in a crystalline environment is studied by SCF calculations on the hole states, with progressively less restrictions on the spatial symmetry of the molecular orbitals. The final wavefunctions are obtained by constructing, from the symmetry broken SCF solutions, wavefunctions that exhibit again the proper transformation properties under the operations of Td. The crystal environment of the [CrO4]2− anion is represented by a point charge model. In contrast with the situation for core hole states, the projection afterwards into Td symmetry is important. The final ionization energies, which are obtained from projected C3v adapted SCF solutions, are reduced considerably (≅3 eV) with respect to the Td ΔSCF results, but the ordering of the states has not changed essentially. The calculated ionization energies compare favourably with results of XPS experiments on Na2CrO4. The evaluation of the energies of projected symmetry broken SCF solutions requires the calculation of hamiltonian matrix elements between determinantal wavefunctions built from mutually non-orthogonal orbital sets. An efficient method for the calculation of such matrix elements is presented.

Effect of metal Ions (Ni2+, Cu2+ and Zn2+) and water coordination on the structure of L-phenylalanine, L-tyrosine, L-tryptophan and their zwitterionic forms

Methods of quantum chemistry have been applied to double-charged complexes involving the transition metals Ni2+, Cu2+ and Zn2+ with the aromatic amino acids (AAA) phenylalanine, tyrosine and tryptophan. The effect of hydration on the relative stability and geometry of the individual species studied has been evaluated within the supermolecule approach. The interaction enthalpies, entropies and Gibbs energies of nine complexes Phe•M, Tyr•M, Trp•M, (M = Ni2+, Cu2+ and Zn2+) were determined at the Becke3LYP density functional level of theory. Of the transition metals studied the bivalent copper cation forms the strongest complexes with AAAs. For Ni2+and Cu2+ the most stable species are the NO coordinated cations in the AAA metal complexes, Zn2+cation prefers a binding to the aromatic part of the AAA (complex II). Some complexes energetically unfavored in the gas-phase are stabilized upon microsolvation.