Computer simulations of polyatomic molecules II. Molecular dynamics studies of diatomic liquids with atom-atom and quadrupole-qiiadrupole potentials

Abstract

The molecular dynamics method has been used to study a model system of 256 homonuclear diatomic molecules governed by intermolecular potentials of the form $U$ $=U^{\text{AA}}$ + $U$$^{\text{QQ}}$, where $U$$^{\text{AA}}$ is an atom-atom potential and $U$$^{\text{QQ}}$ is the quadrupole-quadrupole potential. For $U$$^{\text{AA}}$ both the full Lennard-Jones and the repulsive part only of the Lennard-Jones potential have been used, and $U$$^{\text{QQ}}$ has been used with reduced quadrupole moments, $Q$$^{\ast}$ = $Q$/($\epsilon $\sigma $^{5}$)$^{\frac{1}{2}}$, from 0.0 to 2.0. Diatomics with elongations, $L$$^{\ast}$ = $L$/$\sigma $ (bond length/atom diameter) between 0.33 and 0.63 have been studied, at reduced temperatures $T$$^{\ast}$ = $kT$/$\epsilon $ from 0.98 to 3.48 and reduced densitites $\rho $$^{\ast}$ = $\rho $\sigma $_{e}^{3}$ from 0.522 to 1.043 (where $\sigma $$_{e}$ is the diameter of a sphere having a volume equal to that of the diatomic). Detailed orientational structure in the liquid has been examined by calculating as many as 22 terms in the spherical harmonic expansion of the angle dependent pair distribution function. At short distances these systems exhibit a high degree of angular correlation, which increases with increasing elongation and density. Pair correlation functions calculated from the Lennard-Jones and Lennard-Jones repulsive models are virtually identical, other parameters being equal, and are similar to those for hard diatomics, indicating that both radial and orientational structure are determined mainly by short-range repulsive forces. The addition of a moderately strong quadrupole term to the potential produces dramatic changes in structure and significant changes in thermodynamic properties. A potential with a moderately strong quadrupole term is found to give the correct qualitative features for the structure factor of liquid bromine.