Bond forces and pressure in diatomic liquids

Abstract

Quantum-mechanical considerations indicate that a reasonable classical model for a diatomic system is one in which bond lengths are constant. Two types of such models may be distinguished: rigid models in which the constant length is imposed from the outset as a geometric constraint and flexible models in which the bond is represented by a stiff spring whose force constant becomes infinite. It has been shown by Honnell et al. that the pressure equations derived on the basis of the two models are equivalent. Here we show that this is also the case for the time-averaged bond forces f ^c _b and f ^nc _b, where f ^c _b is the constraint force in the bond of the rigid model or the covalent bond force in the flexible model, while f ^nc _b arises from the intermolecular non-covalent forces projected onto the bond direction. We show that these forces satisfy the relation f ^c _b + f ^nc _b = 2kT/b, with b the bond length, and that the bond-force concept and this relation provide physical insight into the various terms (intermolecular and intramolecular) in the pressure equation. The results of molecular-dynamics calculations for model systems are presented that focus on the bond forces, the relative importance of the intra- and intermolecular contributions to the pressure, and the effect of an attractive portion of the non-covalent site-site potential on these quantities. The implications of these results for the treatment of diatomic liquids by first-order perturbation calculations are discussed.