Prof. Dr. Frank Neese

There are two leading themes pursued by the Neese group: (a) close interaction between theory and experiment in the elucidation of the geometric and electronic structures of transition metal ions in various areas of chemistry and (b) development, improvement and efficient implementation of quantum chemical methods, mainly in the fields of ab initio quantum chemistry and theoretical spectroscopy.

Overall scientific approach pursued in the group.

Transition metal ions play a vital role in bioinorganic chemistry, coordination chemistry, and material science (in particular in the field of molecular magnetism). They show a vast range of fascinating catalytic properties and physical properties which can be probed by sophisticated physical measurements. In order to interpret such measurements it is necessary to combine the experiments with state-of-the-art quantum chemical computations. Many of the properties of interest do not belong to the standard arsenal of quantum chemistry and consequently, an important aspect of this research is the development of new methods for the prediction of physical (foremost spectroscopic) properties of molecules. Since open-shell transition metal compounds belong to the most challenging systems for quantum chemistry, methods that work for such systems are very likely to be successful in other areas of chemistry as well.

Contributions of quantum chemistry to joint experimental and computational studies.

With this background, the group is involved in a number of activities:

• Development of the flexible, user-friendly and efficient large-scale electronic structure package ORCA. This set of programs features a wiede variety of electronic structure methods that are based on Hartree-Fock theory, density functional theory and also semi-empirical methods. The program is a general purpose package and is available free of charge. Its main mission is the prediction of spectroscopic properties but it is efficient for other tasks as well. The program is entirely written in C++ and is essentially fully parallelized. It runs on a wide variety of platforms.

Features and capabilities of the ORCA program.

• Development of new methods for the prediction of magnetic spectra (EPR, Mössbauer and more recently also NMR). In particular, the rather wide variety of properties probed by electron-paramagnetic resonance (EPR) spectroscopy and its modern high-tech variants are being actively pursued (zero-field splittings, g-tensors, hyperfine couplings, quadrupole couplings). These studies are particularly aimed at the field of single-molecule magnets and active sites of metalloproteins. Both ab initio and DFT methods are being pursued. The methods involve treatment of many "small" and complicated often spin-dependent terms in the Hamiltonian (spin-orbit coupling, electron spin-spin coupling, Zeeman effect, scalar relativistic effects).

General approach to the calculation of molecular properties by response theory (see for example Neese, F. (2001) J. Chem. Phys., 115, 11080)

• Development of methods for the prediction of optical and vibrational spectra (absorption, CD, MCD, Raman and in particular resonance-Raman spectroscopy).
  
  
  Example of a calculated absorption and resonance-Raman spectra of a metal-.bridged diradical-complex. (from Petrenko, T.; Ray, K.; Wieghardt, K.; Neese, F. (2006) J. Am. Chem. Soc., 128, 4422-4436)
  
  The main emphasis is on simplified multireference methods that are generally applicable and yields correct results in the presence of near-degeneracies, multiplet effects and spin-orbit coupling.

Example of the calculated d-d spectra of [Ni(H₂O)₆]²⁺ with the SORCI method and comparison to time-dependent density functional theory (Calculations from: Neese F.; Petrenko, T.; Ganyushin, D.; Olbrich, G. (2006) Coord. Chem. Rev., in press; SORCI: Neese, F. (2003) J. Chem. Phys., 119, 9428-9443).

• Development of simplified static and dynamic electron correlation methods. Among those methods, the complete active space self-consistent field (CASSCF) method forms a solid basis for the treatment of dynamic correlation effects in the general case. Simplified single-reference coupled-cluster methods with low-order scaling are developed for the application of high-accuracy methods for large systems.

Basis of low-order scaling single-reference electron correlation methods. The so-called "pair-correlation energies" fall of very quickly with the "separation" of the electrons in localized orbital pairs (unpublished).

• Treatment of environment effects via continuum dielectric approaches, detailed QM/MM calculations and also classical simulations are being pursued by the group.

Example of some calculated spectroscopic parameters (EPR spectra) of a transition metal active site in metalloproteins (Cu²⁺ in platocyanin; from Sinnecker, S.; Neese, F. (2006) J. Comp. Chem.; in press).

• We intensely collaborate with a large number of experimental groups that are involved in the study of metalloproteins, metalloenzymes and model complexes. These studies are directed towards elucidating the geometric and electronic structure of the metallic active sites which perform a fascinating array of catalytic reactions. Emphasis has in the recent past been on high-valent reactive iron sites, metal-radical coupling and enzymes involved in the nitrogen cycle.
• Another intensively pursued collaborative branch of research is the interaction with high-resolution EPR spectroscopists both in the fields of free-radical as well as coordination chemistry research.

Experimental generation and charaterization (X-ray absorption and Mössbauer spectroscopy) of the first model complex of Fe(V) in collaboration with the group of Prof. Wieghardt at the MPI for bioinorganic chemistry in Mülheim (from Aliaga-Alcade, N.; DeBeer George, S.; Bill, E.; Wieghardt, K.; Neese, F. (2005) Angewandte Chemie Int. Ed., 44, 2908-2912)

Elucidation of the photolysis product as a genuine Fe(V) complex with the unexpected ground state S=1/2 (²E) from a combination of spectroscopy and DFT calculations.

Synthesis and characterization of the first genuine Fe(VI) complex. Again, a combination of theory (DFT calculations) and spectroscopy (Mössbauer, XAS) was instrumental in establishing the identity of this unique species (from Berry, J.F.; Bill, E.; Bothe, E.; DeBeer-George, S.; Mienert, B.; Neese, F.; Wieghardt, K.  (2006) Science, in press).