  Screening of a strongly charged macroion by its multivalent counterions can
not be described in the framework of mean field Poisson-Boltzmann (PB) theory
because multivalent counterions form a strongly correlated liquid (SCL) on the
surface of the macroion. It was predicted that a distant counterion polarizes
the SCL as if it were a metallic surface and creates an electrostatic image.
The attractive potential energy of the image is the reason why the charge
density of counterions decreases faster with distance from the charged surface
than in PB theory. Using the Monte Carlo method to find the equilibrium
distribution of counterions around the macroion, we confirm the existence of
the image potential energy. It is also shown that due to the negative screening
length of the SCL, $-2\xi$, the effective metallic surface is actually above
the SCL by $|\xi|$.
