Structural Response of the Water Dimer to an Electric Field

In this paper, we report simulations of the water dimer under a uniform external electric field carried out within the framework of density functional theory (DFT). By gradually increasing the field intensity, we identify a structural transition characterized by an abrupt change in the orientation of the molecular dipole moments relative to the applied field. Below a critical field strength, the system remains in a conventional water dimer configuration. Above this limit, the dimer undergoes a reorientation, leading to a cis-like arrangement associated with a unilateral movement of the donor hydrogen. Vibrational analysis confirms that this field-induced cis configuration corresponds to a stable local minimum. We further analyze the structural and electronic properties of this configuration and propose a semiclassical interpretation of the transition based on the interaction between dipole-dipole interactions and the torque exerted by the external electric field within a classical electrodynamic structure. Hysteresis analysis reveals that the electric field drives the system between distinct local minima on the potential-energy surface, stabilizing a high-dipole structure even after the external field is removed.