Ab initio calculation of optical spectra of liquids: Many-body effects in the electronic excitations of water

We present ab initio calculations of the excited state properties of liquid water in the framework of many-body Green's function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, and the results are averaged over the different configurations. The optical absorption spectra with the inclusion of excitonic effects are calculated by solving the Bethe-Salpeter equation. The insensitivity of screening effects to a particular configuration make these calculations feasible. The resulting spectra, which are strongly modified by many-body effects, are in good agreement with experiments.