Molecular Insights into Optical Spectroscopy of Liquid Water and Ice

Optical spectroscopy is a powerful experimental technique to probe the electronic structure of water. An accurate modeling of optical spectra demands accurate descriptions of both molecular structures and electron-hole excitation processes from first principles. Using the equilibrated molecular structure obtained by the state-of-the-art deep potential path-integral molecular dynamics simulations with the density-functional theory data at the levels of SCAN0 hybrid functional, we compute optical spectra of liquid water and ice by solving the GW-Bethe-Salpeter equation as implemented in the BerkeleyGW package. Our theoretical optical spectrum agrees well with existing experiments. We further assign the peaks with molecular orbitals and identify the molecular motifs for the optical excitons in ice and liquid water.