  An equilibrium theory of classical fluids based on the space distribution
among the particles is derived in the framework of the energy minimization
method. This study is motivated by current difficulties of evaluation of
optical properties in atmospheres of degenerate stars. Present paper focuses on
diluted one-component systems, where the interaction energy is calculated as a
sum of binary contributions. The spatial configuration of the gas is described
in terms of a particle-state variable v which roughly measures the free space
surrounding each particle. The formalism offers a unified treatment of both
thermodynamics and structure of fluids, since it not only provides the state
function (the Helmholtz free energy) of a fluid, but also automatically gives
information on the microstructure of the system (e.g. the nearest-neighbor
distribution function). Equations of state and nearest-neighbor distribution
functions of perfect and hard-body fluids are obtained in straightforward way
in one, two and three dimensions. The formalism is applied to describe the
atomic population in a partially ionized hydrogen gas. Combined and
self-consistent evaluations of atomic populations and internal effects on bound
states are performed in a detailed form. The present theory allow us to resolve
the atomic density at each internal level into groups of atoms which experiment
different perturbation intensities according to the size of their spaces v.
