  By using the Kirkwood formula, the friction coefficient of a solvated
Brownian particle is determined from the integration on time of the
autocorrelation function of the force that the solvent exerts on this particle.
Extensive molecular dynamics simulations show that above a definite size of the
studied systems the value of the integral defining the friction coefficient
goes to a quasi constant value (a plateau) when the upper bound on time
increases. The minimal value of the system size where the integral exhibits
this asymptotic behavior, rises with the Brownian particle size. From the
plateau, a reliable estimate of the friction coefficient is obtained.
