Particle fluctuations within sub-regions of an NN-particle, three-dimensional fluid: Finite-size effects and compressibility

We derive an expression which relates fluctuations in particle number, in the thermodynamic limit with fluctuations within a sub-region of an NN-particle, three-dimensional, simple fluid. The accuracy of our result is tested against simulation results for a three-dimensional, hard-sphere fluid at a low, moderate, and high density state and we find agreement within the uncertainties of the simulation when restrictions associated with the result are enforced. Also, we demonstrate that the bulk isothermal compressibility can be accurately determined directly from fluctuation data obtained from simulations. Our expression yields a quantitative description of the (explicit) size effects associated with fluctuations within sub-regions of an NN-particle system. The approach used in this paper is an extension of the approach used in a previous paper [J.J. Salacuse, Physica A 379 (2007) 372] associated with two-dimensional fluid systems.