A new model for polar molecules is presented based on the concept of a point dipole, displaced from the origin of the dispersion and repulsion forces of the molecule. The second compressibility and dielectric virial coefficients are evaluated numerically for the simplest form of the model, off-centre dipoles located in hard-sphere molecules, and the relevant series expansions given for more general forms of intermolecular potentials. Large enhancements in the magnitude of both types of virial coefficients are found for dipole displacements of about 1 A in small molecules, and negative dielectric virial coefficients are predicted. A comparison with the experimental results available for both single component systems and mixtures lends considerable support to the validity of the model.
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