Angular Pair Correlation Function Research Articles

Monte Carlo study of the angular pair correlation function in a liquid with quadrupolar forces

The angular pair correlation function g( R 12 ω 1 ω 2), giving the probability that two molecules have orientations ω 1 and ω 2 and are at vector separation R 12, has been studied by the Monte Carlo method for a dense liquid. The intermolecular potential model studied consisted of a Lennard-Joes (12,6) potential plus quadrupole—quadrupole interaction. The results are used to test several approximation methods for determining g( R 12ω 1ω 2). While none of these methods works for the largest quadrupole moment tested ( Q * = 1), the perturbation methods give good results for smaller moments.

Angular correlation effects in neutron diffraction from molecular fluids

The structure factor S(q) for a fluid of rigid molecules is related to the angular pair correlation function, g(R 12ω1ω2). Several limiting cases of this relation are considered, and its use as a means of obtaining experimental knowledge of orientation correlations is briefly discussed. A first-order perturbation treatment of S(q) is presented, using as a reference a system of molecules with isotropic forces. The theory is used to predict the effect on S(q) of anisotropic dispersion, overlap and multipole (dipole, quadrupole, octopole, and hexadecapole) forces for linear AA, AB and BAB molecules, and also for tetrahedral AB4 molecules and octahedral AB6 molecules. Comparison with experiment is made for liquid bromine and liquid carbon tetrachloride; for both liquids orientation correlations make a substantial contribution to S(q). The theoretical calculations suggest that the most important anisotropic force contribution to S(q) is probably the quadrupolar force in the case of bromine, and the octopolar force in the case of carbon tetrachloride. The models used in this work for the anisotropic dispersion and overlap forces were not able to account for the observed behaviour of S(q) for these two liquids.