Modeling Core-Ionization with Auxiliary-Field Quantum Monte Carlo

Recent advances in attosecond science have led to important breakthroughs in using X-ray spectroscopy tools to elucidate the structure of molecules and materials. For instance, X-Ray photoelectron spectroscopy (XPS) relies on ionization of the innermost orbitals of a system, being thus employed to study surfaces and interfacial processes. By probing the core electrons of a system, XPS offers unparalleled element-specific spectral signatures, unveiling the nature of the local electronic structure and its chemical environments with outstanding resolution.

From a computational perspective, developing accurate and efficient methods to simulate such processes is essential to help interpret experiments. Auxiliary-field quantum Monte Carlo (AFQMC) has emerged as a promising alternative to traditional electronic structure methods, as it combines the affordable polynomial scaling of DFT with a proper many-body treatment of the electron-electron interaction from wavefunction methods. In this work, we investigate how AFQMC can be used to model XPS spectra of molecular systems, in particular ionizations out of the 1s orbital (K-edge). We discuss how the inclusion of orbital relaxation effects in the trial wavefunction and different orbital-freezing schemes can impact the core-binding energies.