Quantum path-integral molecular dynamics calculations of the dipole-bound state of the water dimer anion

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.