Molecular modeling at high X-ray intensity. Developments, applications and challenges

The advent of X-ray free-electron lasers (XFELs) over the past decade, providing ultraintense femtosecond X-ray pulses, offers novel opportunities for molecular structure investigations. By now, most of the established approaches revolve around diffractive imaging schemes with crystalline samples, where electronic and nuclear radiation damage, caused by the X-rays, must be avoided to achieve the highest possible spatial resolution. My results from ab initio calculations reveal that the induced electronic radiation damage is fundamentally different for soft as compared to hard XFEL pulses. XFEL pulses, however, can also be used to image the atomic building blocks of molecules themselves, by measuring the momenta of atomic ions after the Coulomb explosion triggered by the XFEL pulse. This method called Coulomb Explosion Imaging (CEI) is still in a premature state and a main challenge that remains nowadays, is the inversion of the measured momentum space information into position space. My calculations employing a hybrid quantum-classical approach are in good agreement with recently obtained experimental results, which renders the used toolkit in the long term promising for structural inversions of XFEL-induced Coulomb explosion imaging data. In the last part of this Ph.D. thesis, I derive a charge transfer scheme which is based on quantum mechanical principles. This framework fills a gap, as in other models the time-dependence of electron transfer is often not modeled on a microscopic level.