Quantum simulation studies of singlet and triplet bipolarons in liquid ammonia

The properties of bipolarons in liquid ammonia have been investigated using path integrals and local density functional theory. In both schemes, the singlet state consists of a large ``peanut''-shaped bubble with the peaks in the electron density separated by about 7 Å. However, the triplet state electrons exist in well-separated isolated cavities. This difference in behavior between the singlet and triplet states is consistent with the large drop in the susceptibility observed in dilute alklai metal-ammonia solutions at a concentration of about 1 mol % alkali metal. The adiabatic dynamics of the two states were studied using the Car-Parrinello local density function theory method. In the singlet state, novel hopping events are observed in which the electrons play leap frog over solvent molecules. The dynamics in the triplet state resembled that of two isolated excess electrons. In general, the results are in accord with a wide range of experimental data on metal-ammonia solutions.