THEORY AND SIMULATION OF CENTRAL FORCE MODEL POTENTIALS - APPLICATIONTO HOMONUCLEAR DIATOMIC-MOLECULES

Structure and thermodynamics of fluids made of particles that interact via a central force model potential are studied by means of Monte Car lo simulations and integral equation theories. The Hamiltonian has two terms, an intramolecular component represented by a harmonic oscillat orlike potential and an intermolecular interaction of the Lennard-Jone s type. The potential does not fulfill the steric saturation condition so it leads to a polydisperse system. First, we investigate the assoc iation (clustering) and thermodynamic properties as a function of the potential parameters, such as the intramolecular potential depth, forc e constant, and bond length. It is shown that the atomic hypernetted c hain (HNC) integral equation provides a correct description of the mod el as compared with simulation results. The calculation of the HNC pse udospinodal curve indicates that the stability boundaries between the vapor and liquid phases are strongly dependent on the bond length and suggests that there might be a direct gas-solid transition for certain elongations. On the other hand, we have assessed the ability of the m odel to describe the thermodynamics and structure of diatomic liquids such as N-2 and halogens. To this end we have devised a procedure to m odel the intramolecular potential depth to reproduce the complete asso ciation limit (i.e., an average number of bonds per particle equal to one). This constraint is imposed on the Ornstein-Zernike integral equa tion in a straightforward numerical way. The structure of the resultin g fluid is compared with results from molecular theories. An excellent agreement between the HNC results for the associating fluid and the r eference interaction site model (RISM)-HNC computations for the atom-a tom model of the same fluid is obtained. There is also a remarkable co incidence between the simulation results for the molecular and the ass ociating liquids, despite the polydisperse character of the latter. Th e stability boundaries in the complete association limit as predicted by the HNC integral equation have been computed for different bond len gths corresponding to real corresponding to real molecular liquids. Th ese boundaries appear close to experimental liquid branch of the vapor -liquid coexistence line of the molecular systems under consideration. (C) 1996 American Institute of Physics.