The proposed project will combine the computation of core-level spectra for organic and light-element systems -- particularly molecules relevant to organic photovoltaics (OPVs) -- with the development of methods for L-edge spectroscopies to be carried out primarily in the quantum-chemistry package VeloxChem, where I am an active contributor. The work will focus on simulations of X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and resonant inelastic X-ray scattering (RIXS), with the goal of interpreting experimental measurements as well as extending VeloxChem's capabilities to include not only K-edges, but also L-edges. These spectroscopies provide element-specific and chemically sensitive information on ground-state electronic structure (XPS, XAS) and molecular excited-states (RIXS). For the description of core- and valence-excited states, the project will employ linear-response time-dependent density functional theory (LR-TDDFT), and appropriate DFT-based approaches for core-ionized states and XPS. The functional design of organic molecules for applications in organic electronics or photovoltaics requires understanding how their properties are affected by combining building-block molecules into more complex materials or by complex environments. It is therefore important to accurately characterize molecular materials going from gas-phase building block molecules to more complex materials. In this project, we will focus on a series of small molecules used as building blocks in OPVs, as well as donor-acceptor monomer units and larger oligomers. Part of the allocated resources will be used to calculate XAS and XPS spectra to interpret experimental data obtained by our collaborators (Cesare Grazioli and Oksana Plekan, Elettra synchrotron) for small molecules, as e.g. benzoxazole derivatives, as well as larger materials obtained from these building blocks. Additionally, by computing plausible photodegradation products, the project will also determine spectral features in RIXS and XAS which may be used to identify the photodegradation of donor co-polymers such as the top-performing D18. Identifying the mechanisms of photodegradation is important for OPVs, as such photo-induced chemical modifications strongly reduce the power conversion efficiencies of the corresponding devices. Part of the allocated resource will be used to validate the extension of XAS and RIXS to L-edges, which will allow to compute, e.g., S 2p spectra for the same molecules. The development of L-edge X-ray spectroscopy methods -- which introduce additional complexity, mainly due to spin--orbit coupling -- requires testing and benchmarking across molecules of increasing size and complexity. Here, the allocated resources will be used for validating and benchmarking the implementation against experimental data, e.g. reference [1] for sulphur- and phosphorus 2p spectra (XAS L-edge), or L-edge RIXS spectra reported in references [2] and [3]. [1] Bernes et al., The Journal of Physical Chemistry C, doi.org/10.1021/acs.jpcc.0c03973 [2] Magnusson et al., Physical Review A -- Atomic, Molecular, and Optical Physics, doi.org/10.1103/PhysRevA.59.4281 [3] Såthe et al., Physical Review A -- Atomic, Molecular, and Optical Physics, doi.org/10.1103/PhysRevA.74.062512