# Multi-messenger x-ray science

In the Schrödinger picture of quantum mechanics, the wavefunction appears as the fundamental entity of any quantum system, from which all properties can be extracted. Since for all but the most simple systems an exact form of the wavefunction cannot be obtained, one has to rely on approximations. The field of Quantum Chemistry deals with the development of theoretical models to describe molecular systems. These models rapidly increase in complexity when converging toward an exact solution and comparison between calculated and measured properties can be cumbersome.

An alternative would be to directly use experimental data to extract a given wavefunction. Unfortunately, this can only be done indirectly since the wavefunction is not an observable, hence an underlying model would still be needed. However, one could rely on the experimental data to account for effects not taken into account by the model, e.g electron correlation, spin-orbit coupling, crystal field, etc.

The field of Quantum Crystalography follows this strategy, with a focus on x-ray diffraction (XRD) data as experimental constrain and the Hartree-Fock approximation for the underlying model.

The idea of our project is to generalize this concept to any kind of experimental data and in particular, we which to extend it to x-ray emission spectroscopy (XES) data. We are developing new methods based on the Coupled-Cluster formalism with the aim to build a generalized model for which the calculated wavefunction will "indirectly" be fitted to any kind of experimental data.

Mixing theory and experiment presents many challenges. The project is at the frontier between chemistry, physics, mathematics and informatics, which is reflected it the highly diverse team currently working on it.

Involved group members:

Main collaborators:

Universität Hamburg, Germany

**Christina Brandt, Tram Do**