Forschungsaktivitäten der Arbeitsgruppe

• Electronic correlation in molecules, solids and at surfaces
• Method development and applications

• Quantum chemical wavefunction-based methods and cluster embedding
• Optical excitations, valence and conduction bands

• Geometric and electronic structure of complex materials
• Computational methods in the framework of density functional theory

• Systematic investigations of electron correlation in extended systems like polymers, nanotubes, semi-conductors and ionic crystals using an especially designed incremental correlation treatment based on localized Hartree-Fock orbital.

• Orbital localization (Wannier functions) in periodic systems by POST-SCF methods. Localization of unoccupied orbitals. Development of localizing algorithms for non-orthogonal orbitals.

• Interfacing state-of-the art quantum chemical and solid state computer codes. Development of size-consistency corrections for multi-reference correlation methods.

• Determination of ground state properties of various bulk materials and conjugated polymers including electron correlation. Investigation of the correlation effects on reduced one-particle density matrices (and Compton profiles)

• Quantitative prediction of valence and conduction bands (the energetics of N-1 and N+1 electron states) in non-conduction solids and polymers. Solving the band gap problem inherent to density functional or Hartree-Fock descriptions.

• Development of local wavefunction-based correlation methods. Use of Green function formalisms for one-particle excitations (quasi-particles).

• Studies on vertical optical excitations and the formation of electron-hole pairs (excitons) in crystalline systems.

• Investigation of complex systems like semi-conducting polymers, ordered phases in ceramics, and surface adsorption phenomena. Development of efficient variants of the Kohn-Sham approach to density functional theory.

Total energy calculations of simple compounds can be performed quite routinely with this type of methods by now and are not our main concern any more. Extension of these techniques to other properties (e.g. density matrices) and applications to more challenging materials (such as conducting polymers) is our current interest.

The main activity of our group, however, is the determination of excited state properties, band structures essentially, but recently also excitons.

The following compounds have been investigated in the last few years:

• poly(para-phenylene) - PPP
• trans polyacetylene and polyethylene
• diamond and silicon
• bulk LiF
• HF and H₂ chains