  When non-solid matter (e.g., liquids or gas) is under constant volume V and
density rho (e.g., in rigid box), spatial positions for their constituents are
restricted by these conditions. We recently focus on the role of constraint in
classical statistical thermodynamics, and find how spatial constraint connects
with equilibrium properties for crystalline solids, which has not been
clarified so far. The present study extend the idea to non-solid matter under
NVT ensemble in classical systems. We provide explicit representation of
canonical average of radial distribution function in terms of spatial
constraint, which can be well characterized by a single special microscopic
state on configuration space called projection state for non-solid matter. We
demonstrate that the special microscopic state can be numerically constructed
for a finite number of particles.
