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First-Principles Calculation of Electric Field Gradients and Hyperfine Couplings in YBa2Cu3O7
The local electronic structure of YBa2Cu3O7 has been calculated using
first-principles cluster methods. Several clusters embedded in an appropriate
background potential have been investigated. The electric field gradients at
the copper and oxygen sites are determined and compared to previous theoretical
calculations and experiments. Spin polarized calculations with different spin
multiplicities have enabled a detailed study of the spin density distribution
to be made and a simultaneous determination of magnetic hyperfine coupling
parameters. The contributions from on-site and transferred hyperfine fields
have been disentangled with the conclusion that the transferred spin densities
essentially are due to nearest neighbour copper ions only with marginal
influence of ions further away. This implies that the variant temperature
dependencies of the planar copper and oxygen NMR spin-lattice relaxation rates
are only compatible with commensurate antiferromagnetic correlations. The
theoretical hyperfine parameters are compared with those derived from
experimental data.Comment: 14 pages, 12 figures, accepted to appear in EPJ
First principles study of local electronic and magnetic properties in pure and electron-doped NdCuO
The local electronic structure of Nd2CuO4 is determined from ab-initio
cluster calculations in the framework of density functional theory.
Spin-polarized calculations with different multiplicities enable a detailed
study of the charge and spin density distributions, using clusters that
comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated
by two different approaches and the resulting changes in the local charge
distribution are studied in detail and compared to the corresponding changes in
hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is
investigated in detail and good agreement is found with experimental values. In
particular the drastic reduction of the main component of the EFG in the
electron-doped material with respect to LaCuO4 is explained by a reduction of
the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical
shieldings at the copper nucleus are determined and are compared to results
obtained from NMR measurements. The magnetic hyperfine coupling constants are
determined from the spin density distribution
Ground state properties of heavy alkali halides
We extend previous work on alkali halides by calculations for the heavy-atom
species RbF, RbCl, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, and RbI. Relativistic
effects are included by means of energy-consistent pseudopotentials,
correlations are treated at the coupled-cluster level. A striking deficiency of
the Hartree-Fock approach are lattice constants deviating by up to 7.5 % from
experimental values which is reduced to a maximum error of 2.4 % by taking into
account electron correlation. Besides, we provide ab-initio data for in-crystal
polarizabilities and van der Waals coefficients.Comment: accepted by Phys. Rev.
THEORETICAL STUDIES OF BILIPROTEIN CHROMOPHORES AND RELATED BILE PIGMENTS BY MOLECULAR ORBITAL AND RAMACHANDRAN TYPE CALCULATIONS
Ramachandran calculations have been used to gain insight into steric hindrance in bile
pigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as
compared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the
equilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures
and transitions have then been studied by molecular orbital calculations for several conjugation
systems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and
tautomeric forms. For these different chromophores some general trends can be deduced. For instance,
for a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully
extended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the
predicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions
between the different n-systems, both in the transition energies and the charge distribution, which
can be related to their known differences in spectroscopic properties and their reactivity
Correlation effects in ionic crystals: I. The cohesive energy of MgO
High-level quantum-chemical calculations, using the coupled-cluster approach
and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m
clusters embedded in a Madelung potential. The results of these calculations
are used for setting up an incremental expansion for the correlation energy of
bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal
is recovered. It is shown that only 60% of the correlation contribution to the
cohesive energy is of intra-ionic origin, the remaining part being caused by
van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure
Ground-state properties of rutile: electron-correlation effects
Electron-correlation effects on cohesive energy, lattice constant and bulk
compressibility of rutile are calculated using an ab-initio scheme. A
competition between the two groups of partially covalent Ti-O bonds is the
reason that the correlation energy does not change linearly with deviations
from the equilibrium geometry, but is dominated by quadratic terms instead. As
a consequence, the Hartree-Fock lattice constants are close to the experimental
ones, while the compressibility is strongly renormalized by electronic
correlations.Comment: 1 figure to appear in Phys. Rev.
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