# Regensburg 2002 – wissenschaftliches Programm

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# O: Oberflächenphysik

## O 40: Oxide und Isolatoren (II)

### O 40.7: Vortrag

### Freitag, 15. März 2002, 12:45–13:00, H45

**An ***ab initio*** embedded cluster study of optical excitations below the gap of a NiO(001) surface** — •Khompat Satitkovitchai and Wolfgang Hübner — Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany

An embedded cluster approach was applied to study the
electronic excitations on the NiO(001) surface. Using the valence
Los-Alamos
basis set and effective core potentials, quantum chemical calculations
of a
small cluster (*NiO*_{5})^{8−} were done starting at the
Hartree-Fock
(HF) level of theory. We have begun with HF theory for calculating
ground-state
properties to provide some insight of electronic structure and
excitation. We
estimate the band gap and excitation spectra using single excitation
Configuration Interaction (CI-Singles) technique. Our HF results yield
a
charge-transfer (CT) gap of 7.6 eV, which is much higher than the
experimental
value of bulk NiO (4.3 eV) as expected on the theoretical HF level but
yet can
be a good starting point to allow for the investigation the low lying
d-d
intragap multiplet excitations. We then demonstrate the electron
correlation
effects on the d-d transitions at several levels of *ab initio*
correlated theory (CID, CISD, CCD, CCSD, CCSD(T), QCISD, QCISD(T)). The
electron correlation tends to decrease the magnitude of excitation
energies.
Only at our highest level theoretical methods such as CCSD(T) and
QCISD(T), the
d-d transition energies (0.53, 1.17, 1.21 and 1.84 eV) are found to
compare well
with the experimental data.