Insights into the structure of simple liquids are presented from analysis of the effect of the range of interatomic forces on the multidimensional potential energy surfaces of bulk material and clusters. An understanding al the microscopic level is provided of how the liquid phase is destabilized in systems with very short-range interparticle forces. For small clusters bound by long-range interatomic forces, the lowest energy minimum has an amorphous structure typical of the liquidlike state. This suggests an explanation for the transition from electronic to geometric magic numbers (structures of special stability) observed in the mass spectra of sodium clusters.