  Theoretical optical and x-ray spectra of model structures of water and ice
are calculated using a many-body perturbation theory, Bethe-Salpeter equation
(BSE) approach implemented in the valence- and core-excitation codes AI2NBSE
and OCEAN. These codes use ab initio density functional theory wave functions
from a plane-wave, pseudopotential code, quasi-particle self energy
corrections, and a BSE treatment of particle-hole interactions. The approach
improves upon independent-particle methods through the inclusion of a complex,
energy-dependent self energy and screened particle-hole interactions to account
for inelastic losses and excitonic effects. These many-body effects are found
to be crucial for quantitative calculations of ice and water spectra.
