Structure of Dielectric Fluids. I. The Two-Particle Distribution Function of Polar Fluids

A molecular fluid of identical molecules with a rigid dipole moment in an arbitrarily shaped volume is considered. The volume may or may not be embedded in a dielectric continuum. It is shown that when an arbitrary external field is applied, the constitutive relation P=(ε−1)/4 πE between the local polarization and the local macroscopic electric field is valid under some completely acceptable restrictions. In the establishment of this relation a crucial role is played by a long-range part of the two-particle correlation function, for which an explicit expression is obtained. One term in this long-range part is explicitly dependent on the shape of the sample volume and on the surroundings. The resulting dielectric constant of the molecular fluid can be formally expressed in only the local interactions of the molecules and is thus independent of the surroundings and the shape of the sample.