Equilibrium properties of molecular fluids with charge distributions of quadrupolar symmetry

Abstract

Molecular dynamics simulations are reported for dense fluids of molecules with discrete charge distributions of quadrupolar symmetry. The nonelectrostatic parts of the intermolecular potentials were modeled by Lennard‐Jones site–site interactions with the two sites in each molecule separated by 0.547σ, where σ is the distance scaling parameter in the Lennard‐Jones function. The charge distributions were comprised of a double negative charge at the molecular center of symmetry plus two single positive charges placed on the symmetry axis at equal distances from the center. The magnitudes and separation distances of the charges were varied so that changes in the thermodynamic, quasithermodynamic (mean square force and torque), and structural properties of the fluid could be determined at a fixed quadrupole moment. At finite charge separation distances, the multipole expansion yields ideal quadrupole–quadrupole plus higher order interactions. It is shown that the observed changes in thermodynamic properties can be accurately calculated by including the next highest multipole interaction terms in the series; namely, the ideal quadrupole–hexadecapole energies (plus a small contribution from the hexadecapole–hexadecapole terms at the largest charge separations). A low order perturbation theory calculation of the changes in structural properties is also presented; it is found not to give satisfactory values for the observed changes in radial distribution function with charge separation distance.