Research in the Neugebauer Group
We develop focused quantumchemical methods and apply quantum chemistry to a wide range of topics:
Our Research Areas:

Subsystem DensityFunctional Theory and FrozenDensity Embedding

FirstPrinciples ElectronicStructure Calculations for Proteins
Subsystem DensityFunctional Theory and FrozenDensity Embedding
Quantum chemical methods suffer from an unfavorable scaling of computer time with the system size, e.g., the number of atoms in a molecular system. The idea of subsystem DFT is to circumvent this problem by decomposing the electron density into subsystem contributions. This has a number of advantages, both from a conceptual point of view and for the computational effort in applications to large molecular systems. FrozenDensity Embedding (FDE) can be regarded as a special variant of subsystem DFT in which the properties of a certain active (embedded) subsystem in a larger environment can be determined.
Our group develops and FDE and subsystem DFT methods for the investigation of the electronic structure and properties of molecular aggregates. Recent aspects of this work include:
 Potentialenergy surfaces from sDFT: We benchmark existing approximations within sDFT and try to improve PESs from sDFT. We have developed analytical gradient implementations for sDFT in the ADF package for ground and excited states.
 "Exact" embedding: We develop, implement, and apply "exact" embedding strategies based on potential reconstruction and projection. We have recently shown that external orthogonality is not a formal requirement for sDFT to be exact.
 Embedding correlated wavefunctions: We work on WFinDFT embedding approaches for higheraccuracy descriptions of molecules in complex environments. Together with our collaboration partners, we have implemented WFinDFT approaches for wavefunction methods such as DMRG, CASSCF, CASPT2, CC2/CCSD/CC3, and QMC. We have rederived expressions for excitation energies including statespecific embedding potentials for WFinDFT embedding, and addressed question related to the nonorthogonality of embedded wavefunctions with such statespecific potentials.
 Molecular properties from sDFT: We have derived and implemented a linearresponse subsystem TDDFT for the calculation of excited states (see also below) and other response properties (UV/Vis spectra, circular dichroism, frequencydependent polarizabilities, optical rotatory dispersion)
 sDFT as a constrainedDFT variant: We have shown that sDFT with approximate nonadditive kineticenergy functionals can pragmatically be used to converge to quasidiabatic electronic states with localized charge or spin. This allows, for example, to study charge transfer based on sDFT.
FirstPrinciples ElectronicStructure Calculations for Proteins
Related to sDFT (see above), we employ the socalled 3partitioning FDE (3FDE) approach for electronicstructure calculations on proteins. 3FDE can be thought of as a sDFT with buffer regions, which allow to cut covalent bonds. The "dangling bonds" are satured with a buffer fragment (capping group). The electron densities of the capping groups (combined to cap molecules) are subsequently subtracted from the sumofcappedfragment density to arrive at a correct total density. Our contributions to this field include:
 Calculations of pigment site energies in a 3FDE environment (including relaxation effects)
 A benchmark of electron densities from 3FDE (avoiding failures of KSDFT)
 A local variant of the COSMO model (needed for modeling protein solvation)
Predicting Magnetic Properties
Part of our research addresses the magnetic properties of organic radicals. In particular, we are working on the following topics:
 Blackbox prediction of macroscopic magnetism: We have recently implemented a blackbox version of the socalled firstprinciples bottomup approach, which allows to predict magnetic susceptibilities and heat capacities as a function of temperature based on a crystal structure for crystalline organic radicals.
 Modeling ESR spectra: We simulate ESR spectra of organic radicals based on electronicstructure theory.
 Spindensity distributions: We are working on accurate yet efficient methods for the prediction of spin densities of organic systems.
Quantum Chemistry for Photosynthetic Systems
The aim of this research project is the development of quantum chemical methods that can be applied in studies of natural and artificial photosynthetic systems. These methods are used to study the mechanisms of initial steps in photosynthesis, namely absorption of light by pigment molecules, energy transfer to a reaction center, and initial chargeseparation steps.
Recent efforts in this project concern:

Proteinpigment interactions and exciton couplings: We have studied excited states of pigments and pigment networks in (essentially) complete natural lightharvesting antennae such as the FennaMatthewsOlson complex

Screening effects on energy transfer: We have developed explicit quantum chemical methods to include environmental screening effects on excited states and excitonic couplings as needed in energytransfer model theories

Charge and spindensity distributions in photosynthetic reaction centers: We have conducted a benchmark study on spin densities for radical cations of photosynthetic pigment models. In addition, we have studied environmental effects on the spindensity asymmetry in the special pair of purple bacteria, and provided support for experimental studies on the spin density in photosynthetic reaction centers.

Resonance Raman studies on photosynthetic systems: We have used (selective) theoretical resonance Raman approaches to unravel structural details of spheroidene in the reaction center of Rhodobacter spheroides and to investigate the reaction coordinate for the initial charge separation in the special pair of bacterial reaction centers
Efficient Algorithms for ExcitedState Calculations
Excitedstate calculations for molecules in solvents or adsorbates, but also for core excitations, often suffer from the fact that the computational effort is large, not only because of the system size, but also because many excited states need to be calculated, many more than actually of interest. We are therefore interested in selective excitedstate methods applicable to large systems.
Recent developments and studies include:

Wavefunction/DFT embedding schemes for excitation energies with statespecfic embedding potentials

Selective excitedstate algorithms including applications to adsorbates, solvated systems, and core excitations

Vibronicstructure tracking (VST) for selective determination of vibrational normal modes with high impact on the vibronic structure

Subsystem TDDFT for electronic excitations including work on the corresponding TammDancoff approximation and applications to networks of coupled chromophores

3FDETDDFT for excitations of pigments in proteins (see also above)
Theoretical OnSurface Chemistry
We have recently started looking into onsurface chemistry phenomena in close connection to experiment. This mainly includes
 Structural characterization of molecules on surfaces
 Energy profiles and transitionstate location for onsurface reactions
 Participation of surface metal atoms in those reactions
Theoretical Vibrational Spectroscopy
Our group is involved in the development of the massively parallel theoretical vibrational spectroscopy package MoViPac, which comprises the two programs SNF (for standard frequency analyses) and Akira (for modetracking calculations). Previous work concerned special types of vibrational spectroscopy such as vibrational circular dichroism (VCD), resonance Raman (RR), and vibrational resonance Raman optical activity (VRROA). Methodologically, we have contributed selective algorithms for tracking characteristic motions, highintensity vibrations, or vibrations with large impact on the vibronic structure of UV/Vis spectra.
Program references, descriptions and illustrative applications can be found in:

SNF: efficient frequency analysis for massively parallel computations [J. Neugebauer, M. Reiher, C. Kind, B.A. Hess, J. Comput. Chem. 23 (2002), 895]

Akira: Modetracking of preselected molecular vibrations in large systems [M. Reiher, J. Neugebauer, J. Chem. Phys. 118 (2003), 1634]

MoViPac: Vibrational Spectroscopy with a Robust MetaProgram for Massively Parallel Standard and Inverse Calculations [T. Weymuth, M. Haag, K. Kiewisch, S. Luber, S. Schenk, C. Jacob, C. Herrmann, J. Neugebauer, M. Reiher, J. Comput. Chem. 33 (2012), 2186]