Anomalous Water Optical Absorption: Large-Scale First-Principles Simulations.

The optical spectrum of water is not well-understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation of gas-phase water monomers, which is contrary to the behaviour expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlation, finding that condensation leads to delocalisation of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]