# Research topics

## Natural and artificial photosynthesis

### Biological water oxidation

Photosynthetic organisms capture the energy of sunlight and convert it into chemical energy by synthesizing carbohydrates, a process that sustains life on Earth as we know it. The main engine of the photosynthetic apparatus is the enzyme **Photosystem II** (see here for a recent review). At its heart is the **oxygen-evolving complex** (OEC), a manganese-calcium cluster that catalyzes the oxidation of water into molecular oxygen, protons and electrons, providing the reducing equivalents subsequently employed in carbon fixation. Elucidating the structure and function of the OEC is a fundamental step towards developing bioinspired systems for artificial photosynthesis, potentially paving the way towards sustainable production of **solar fuels**.

Our research aims at understanding the electronic structure, catalytic function, and regulation mechanisms of Photosystem II in general and of the OEC in particular. This is achieved through a combination of experimental approaches with modern **multiscale computational methods** that range from whole-protein molecular dynamics to highly accurate quantum chemical calculations of the electronic structure of redox-active sites. An important component of our research is the development of theoretical methods for the prediction of spectroscopic observables for a given structural model, such as those derived from magnetic resonance (EPR, ENDOR, etc) and X-ray spectroscopies.

Some of our contributions to this area include the detailed understanding of the structural fluxionality of the OEC in the S_{2} state, the elucidation of the electronic structure of the cluster in the S_{3} state, and the development of experimentally consistent computational models for all stable states of the catalytic cycle.

### Synthetic water splitting catalysts

Development of artificial water oxidation systems that can be deployed on a large scale is challenging because of the high and often conflicting demands in terms of efficiency, stability, and cost. We participate in research networks that aim to develop such catalysts, both molecular and heterogeneous, based on earth-abundant metals such as cobalt and manganese. Our group develops and applies theoretical methods that enable us to extract information on the local geometric and electronic structure of active sites in order to understand how they operate at the atomic level.

## Transition metal complexes: magnetism, spectroscopy, reactivity

### Polynuclear magnetically coupled systems

Unique electronic and chemical properties arise when metal ions with unpaired electrons interact with each other. We have extensive experience in dealing with such systems using a wide array of theoretical approaches and computational methodologies, ranging from broken-symmetry DFT to DMRG. The ability to correctly describe the electronic structure of these magnetically coupled systems has been essential in the study of not only the oxygen evolving complex, but also of many synthetic complexes, metal oxide surfaces, and single-molecule magnets.

### Methodologies for theoretical spectroscopy

In collaboration with other groups at the MPI-CEC, we develop and implement theoretical methods for the calculation of spectroscopic parameters in transition metal complexes, particularly for systems that contain several sites with unpaired electrons. These methods are key for interpreting complex experimental data and for devising models that explain the structure and function of inorganic and bioinorganic catalysts.

Examples of such approaches include a spin-projection method developed to calculate **hyperfine coupling constants** for oligonuclear metal clusters of arbitrary shape and nuclearity, and a method to compute local **zero-field splitting** parameters in polynuclear clusters with a local complete active space configuration interaction (L-CASCI) approach.

## Accurate energetics in solution

The prediction of accurate redox potentials, acidity constants, and reaction energetics in solution or in the dynamic environment of a protein remains a challenge for computational chemistry when open-shell systems such as transition metal ions are involved. To achieve consistently high accuracy regardless of the electronic complexity of the system under study, it is often necessary to move beyond density functional theory and continuum solvation models. To address this challenge we are developing approaches that combine cutting-edge quantum chemical techniques such as the**local pair natural orbital**(LPNO) approaches to coupled cluster theory, with explicit solvation and

*ab initio*molecular dynamics.

## Development of all-electron scalar relativistic basis sets

### An alternative to pseudopotentials

Computational investigations of chemical systems containing heavy elements typically employ effective core potentials, which reduce the size of the computational task while providing an easy, if approximate way to account for relativistic effects. However, ECPs have their drawbacks; for example, they cannot be used for properties that depend on the electron density near the nucleus, or for reliable topological analysis of electron densities. To circumvent the limitations of ECPs it is necessary to have all-electron basis sets that allow efficient calculations with the popular scalar relativistic Hamiltonians, such as the Zeroth Order Regular Approximation (**ZORA**), and the Douglas-Kroll-Hess (**DKH**) approach.

An answer is our family of **Segmented All-electron Relativistically Contracted (SARC) basis sets**, constructed specifically for DFT calculations in conjunction with the DKH2 and ZORA Hamiltonians. The SARC basis sets are relatively small CGTO sets of TZVP quality that follow a segmented (as opposed to general) contraction. They are a good choice for routine computational studies of large molecules containing heavy elements and their performance has been tested for both atomic and molecular systems with respect to structural and energetic properties.

> Visit the SARC page for more information and downloads.