  In the long-wavelength limit k=0, the response function has been investigated
with respect to the external and internal fields which is expressed by the
external and internal conductivity, respectively. Molecular dynamics (MD)
simulations are performed to obtain the current-current correlation function
and the dynamical collision frequency which are compared with analytical
expressions. Special attention is given to the dynamical collision frequency
and the description of plasma oscillations in the case of k=0. The relation
between the external and internal conductivity and to the current-current
correlation function is analyzed.
