The equilibrium structure of the negatively charged water dimer (H2O)(2)(-) has been studied using the path-integral molecular dynamics simulation. All the atomic motions as well as the excess electron were treated quantum mechanically, employing a semi-empirical model combining a water-water interatomic potential with an electron-water pseudopotential. It is demonstrated that the molecular structure of (H2O)(2)(-) is more flexible than that of (H2O)(2); both the donor switching and donor acceptor interchange can more effectively occur in (H2O)(2)(-) than in (H2O)(2). We conclude that this floppy character is a result of the breakdown of the adiabatic Born-Oppenheimer picture. (C) 2003 Elsevier B.V. All rights reserved.