Thermodynamic and structural properties of liquids modelled by ‘2-Lennard-Jones centres’ pair potentials

Molecular dynamics calculations have been carried out for model liquid systems of N (=108 or 256) molecules interacting through two Lennard-Jones (12–6) centres coinciding with the positions of the atomic masses (the ‘atom-atom’ pair potential). The objectives were (a) to study the dependence of the properties on the molecular anisotropy defined by the reduced distance l*=l/σ between the centres in the range 0·5–0·8; and (b) to compare the computed quantities with those of real liquids (F2, Cl2, Br2, CO2). This paper deals with thermodynamic and structural features. Time-dependent correlations will be treated in a future communication. In the liquid region not too far from the triple point the energy and pressure isochores are well represented by straight lines, the slopes of which increase with density and anisotropy. Thermodynamically consistent expressions for the energy and pressure as functions of density and temperature have been obtained for each system. With Lennard-Jones parameters adjusted so as to secure the best overall fit, the agreement between experimental and computed thermodynamic properties is very satisfactory for F2 (l*=0·505), quite good for Cl2 and Br2 (l*=0·608–0·63), but rather poor for CO2 (l*=0·793). The ‘interatomic distances’ are close to the experimental values. The static structural correlations are discussed in terms of the pair-correlation functions (pcf) g A(r*) for the separation between ‘atoms’, the first few functions gll'm (R*) which arise from the expansion of the g(R*, θ1, θ2, φ12) in spherical harmonics, and the pcf's for certain special near-neighbour configurations. The computed atom-atom structure factor is compared with the experimental data for liquid Br2. Mean square forces and torques have been evaluated and are related to some experimental results.