Abstract
This work emphasizes the potential of hybrid functionals (B3LYP) in the study of
a wide range systems. This functional by take advantage of the exact Hartree-Fock
exchange, can be used for both molecular and crystalline systems. For example, strongly
correlated systems are studied by B3LYP, successfully.
The electronic structure of the PAHs has been studied with the help of B3LYP hybrid
density functionals. Using the ∆SCF method, electron binding energies have been
determined which affirm, specify and complement the experiment results measured by
UPS. Symmetry properties of molecular orbitals are analyzed for a categorization and
an estimate of the related signal strength. While σ-like orbitals are difficult to detect by
UPS measurements of condensed films, calculations provide a detailed insight into the
hidden parts of the spectra. Afterward, the π − π complexes formed by several donor
and acceptor molecules based on polycyclic aromatic hydrocarbons have been studied.
For the charge transfer complexes, the DFT calculation provide minimum one the
potential energy surface. This attraction is caused by Coulomb interactions. However,
the attraction based on Coulomb interaction is not the strongest interaction in CTCs.
The vdW corrections improve the intermolecular distances as well as the binding energy.
Decreasing the intermolecular distances leads to large shifts in the HOMO and LUMO
energies. For the crystalline systems Rb4O6 and FeSe have been studied as a correlated
systems. In the case of Rb4O6 charge ordering and correlations leads to an insulating
ground state in this system. The hypothetical pressure dependent phase diagram was
studied. Applying pressure leads to increasing band-width. At about 75 GPa, W
exceeds U the system becomes homogeneous mixed valent with a fractional occupation
of the p-orbitals and finally for at 160 GPa W>> U the system becomes metallic. In
the case of FeSe there is a correlated and insulating phases at high pressures, while
at low pressure the system is superconducting. The electronic structure calculation
with the hybrid functional B3LYP leads to the correct semiconducting ground state for
NiAs- and MnP-type structure of FeSe. The role of correlation, stoichiometry and the
borderline to magnetism are discussed. In particular, it is shown that the NiAs-phase
structure exhibits strong local correlations which lead to a semiconducting state within
the broad pressure range.
To download the dissertation click on the link below:
http://ubm.opus.hbz-nrw.de/volltexte/2011/2670/pdf/doc.pdf
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