The one-electron density of states of liquid water computed from an ab initio molecular dynamics trajectory is analyzed in terms of interactions between effective molecular orbitals localized on single molecules. These orbitals are constructed from the occupied extended (Kohn-Sham) orbitals using the maximally localized Wannier function method. Band positions are related to average orbital energies. The width of a band is resolved into contributions from thermal fluctuations in the orbital energies and the electronic broadening due to intermolecular coupling. It is found that the thermal and electronic broadening are of comparable magnitude with electronic broadening being the leading effect. (C) 2003 Elsevier Science B.V. All rights reserved.