  Recent atomic force microscopy (AFM) can measure force curves between a probe
and a sample surface in solvent. The force curve is thought as the solvation
structure in some cases, because its shape is generally oscilltive and pitch of
the oscillation is about the same as diameter of the solvent. However, it is
not the solvation structure. It is just only a mean force between the probe and
the sample surface. A theoretical relation between the mean force and the
solvation structure is not clearly known. Therefore, the relation must be
elucidated theoretically to deepen understanding of the mean force measured by
the AFM. In this letter, we briefly explain the relation and a method for
comparing the measured mean force and the solvation structure (that obtained by
a simulation, a liquid theory, or a x-ray reflectivity) by using basic
statistical mechanics of liquid.
