The isotropic-nematic transition in hard Gaussian overlap fluids

We report a study of the isotropic-nematic (I-N) transition in fluids of mo lecules interacting through the hard Gaussian overlap (HGO) model. This mod el is computationally simple and shears some similarities with the widely u sed hard ellipsoid (HE) fluid. The I-N coexistence properties of HGO fluids have been accurately determined by combining different simulation techniqu es, including thermodynamic integration and Gibbs-Duhem integration for var ious molecular elongations, kappa, in the range 3 less than or equal to kap pa less than or equal to 10. The accuracy of the Gibbs-Duhem integration sc heme has been independently assessed by locating the I-N transition using t he Gibbs-ensemble simulation technique for the largest elongation considere d here (kappa =10). The simulation results are analyzed within the context of the simplest version of the decoupling approximation as introduced by Pa rsons and Lee. The agreement between theoretical predictions and simulation data might be considered satisfactory, particularly for large nonsphericit ies. A comparison with the simulation results for the I-N properties of HE fluids shows that there exist large quantitative differences between HGO an d HE fluids in this region, and this is ascribed to the larger volume exclu ded by a pair of HGO molecules compared to that of HE. In the light of the results presented here, approximating the distance of closest approach, or the excluded volume in the HE model by the corresponding expressions borrow ed from the Gaussian overlap approximation-as is implemented in some theore tical descriptions of the HE fluid-does not seem to be appropriate. (C) 200 1 American Institute of Physics.