Multireference Polarizable Embedding for Core-Level Spectroscopy in Aqueous Solution: XPS and XAS of Glycine
Core-level spectra in solution are governed by the interplay of core-hole relaxation, electron correlation, specific solute--solvent interactions, and environmental polarization. We report the first application of RASSCF--RASPT2 combined with the fluctuating-charges polarizable embedding to X-ray photoelectron (XPS) and X-ray absorption (XAS) spectroscopy in aqueous solution. The protocol couples a multireference treatment of core-ionized and core-excited states with molecular-dynamics sampling, and is assessed for the carbon and nitrogen K-edges of glycine in the gas phase and in water. The calculated spectra are compared with experiment and with fixed-charges electrostatic embedding. Gas-phase ionization and absorption features are accurately reproduced. In solution, configurational averaging captures inhomogeneous broadening, while XPS peak positions, relative intensities, and widths depend strongly on the solvent parametrization. The best-performing polarizable model improves the spectral profile, although aqueous ionization energies remain overestimated. For XAS, the carbon K-edge is reproduced close to the experimental energy scale, whereas the nitrogen K-edge line shape is recovered after rigid alignment. These results support multireference polarizable embedding as a promising framework for core-level spectroscopy in solution.