X-ray Absorption Spectral Signatures of Molecular Environments in Liquid Water, Crystalline and Amorphous Ice

We investigate the oxygen K-edge x-ray absorption spectra of liquid water, hexagonal ice, ice VIII, and high- and low-density amorphous ices based on approximate approach to the Bethe-Salpeter equations for electron-hole excitations. In the excitations, the core hole is treated as a frozen O 1s state, and the excited electron is described by a self-consistent quasiparticle wavefunction obtained by diagonalizing the static Coulomb-hole plus screened-exchange self-energy operator. Molecular configurations are generated by path-integral deep-potential molecular dynamics with nuclear quantum effects, and the neural-network potential is trained on density function theory data using the SCAN meta-GGA functional. The calculated spectra reproduce, in quantitative agreement with experiment, both the peak positions and spectral widths, and consistently capture the pre-edge, main-edge, and post-edge features across liquid and solid phases.