  The problem of surface effects at a fluid/force field boundary is
investigated. A classical simple fluid with a locally introduced field
simulating a solid is considered. For the case of a hard-core field, rigid,
exponential, realistic, and macroscopically smooth boundaries are examined.
  Two approaches to this problem are analyzed. With some degree of
arbitrariness, they can be referred to as "adsorption" vs "surface tension" or
"cluster expansion" vs "pressure tensor".
  The "adsorption" approach is used to obtain a series in powers of the
activity for gamma. For Mayer-type expansion the integrals of the Ursell
functions contain factors which depend on the particle/wall interaction
potential. In the case of a hard wall, the coefficients of the series reduce to
the first moments of the Ursell functions taken over certain regions.
  The "surface tension" approach is used to expand the Kirkwood-Buff formula
for gamma to the arbitrary localization of the dividing surface. "The surface
tension coefficient" breaks up into the term proportional to the Henry
constant, depending on the dividing surface position, and universal nonlinear
surface coefficient.
  It is shown that the derivative of the tangential component of the pressure
tensor with respect to the chemical potential coincides with the near-surface
number density on average over the transition region, that has two
consequences.
  Firstly, it proves complete identity between "tension" and "adsorption"
approaches in the domain of their existence.
  Secondly, it gives the near-surface virial expansion, which determines the
exact equation of state of near boundary "two-dimensional" fluid. The
tangential component of the pressure tensor averaged over the transition region
plays the role of pressure, and the average number density - the role of number
density.
