First-principles molecular dynamics of liquid rubidium at low density

We performed a first-principles molecular dynamics study of liquid Rubidium in a thermodynamic state in which experimental evidence of a structural change related to the metal–non-metal transition has been found (specifically, the appearance of a side peak in S(q, ω)). Analysis of the trajectories described by the particles during the simulation showed no indication of a transition from a monoatomic to a molecular state. However, we did detect cases of transient correlation among the motions of three ions, in which two ‘terminal’ ions separated by about 10 Å performed mutually out-of-phase vibrations with respect to a ‘central’ ion about half-way between them, the frequency of the vibrations being close to that of the reported inelastic peak in S(q, ω). These transient trimeric structures lasted for several vibration periods. Results for the static structure agree well with experiment. The computed mean square displacement, velocity autocorrelation function and diffusion constant are also reported.