For a large region of dense fluid states of a Lennard-Jones system, we have
calculated the friction coefficient by the force autocorrelation function of a
Brownian-type particle by molecular dynamics (MD). The time integral over
the force autocorrelation function showed an interesting behavior and the
expected plateau value when the mass of the Brownian particle was chosen to be
about a factor of 100 larger than the mass of the fluid particle. Sufficient
agreement was found for the friction coefficient calculated by this way and that
obtained by MD calculations of the self-diffusion coefficient using the common
relation between these coefficients. Furthermore, a modified friction coefficient
was determined by integration of the force autocorrelation function up to the
first maximum. This coefficient can successfully be used to derive a reasonable
"soft part" of the friction coefficient necessary for the Rice-Allnatt
approximation for the shear viscosity of simple liquids