Variational approach to Yukawa fluids. II. Instantaneous elastic moduli and sound velocities.

The variational approach based on the Bogoliubov inequality using the fluid of hard spheres as a reference system is implemented to evaluate instantaneous shear, bulk, and longitudinal elastic moduli, as well as related sound velocities of Yukawa fluids. The remarkable accuracy of this method is documented. In addition, we evaluate the adiabatic sound velocity from an appropriate equation of state and discuss its relation to the longitudinal and bulk sound velocities obtained from the corresponding instantaneous elastic moduli. The transition between weakly coupled and strongly coupled regimes is analyzed in detail.