Quantum simulation studies of metal-ammonia solutions

Metal-ammonia solutions are examined from the insulating to the conducting regime using three different microscopic models. In model I, the ammonia molecules are treated via a classical point charge model and the cations as a positive neutralizing background. In model II, the ammonia solvent is made fully polarizable and the cations are again, treated as a positive background. Finally, in model III, the solvent is taken to be fully polarizable and the ions, here lithium, are explicitly included. At T=260 K, the following picture emerges of the electronic states as a function of metal/electron concentration: The dilute solution behaves like an electrolyte in which the electrons exist as polarons, on average spherical states localized in solvent cavities, far from the counterions. At 1 mole percent metal (MPM), the electrons spin pair, forming peanut-shaped species called bipolarons. At slightly higher concentration, 2 MPM, the electronic states oscillate (moderated by solvent fluctuations) between dimers of bipolarons and connected tubular states. At 9 MPM, a good liquid metal is formed in which the electron density forms tubular extended states. These observations are in good agreement with experiment. The quantitative differences between the three models are discussed. In general, the effects of solvent polarizability are found to be rather small. However, the explicit inclusion of the cations is found to slightly increase the metallic character of the solution at 9 MPM.