Dynamics and configurational entropy in the Lewis-Wahnström model for supercooled orthoterphenyl

We study thermodynamic and dynamic properties of a rigid model of the fragile glass-forming liquid orthoterphenyl. This model, introduced by Lewis and Wahnström in 1993, collapses each phenyl ring to a single interaction site; the intermolecular site-site interactions are described by the Lennard-Jones potential whose parameters have been selected to reproduce some bulk properties of the orthoterphenyl molecule. A system of $N=343\mathrm{}$ molecules is considered in a wide range of densities and temperatures, reaching simulation times up to $1\hspace{0.5em}\mu s.\mathrm{}$ Such long trajectories allow us to equilibrate the system at temperatures below the mode coupling temperature $T_c$ at which the diffusion constant reaches values of order ${10}^{-10}\hspace{0.5em}{\mathrm{cm}}^2/\mathrm{s}$ and thereby to sample in a significant way the potential energy landscape in the entire temperature range. Working within the inherent structures thermodynamic formalism, we present results for the temperature and density dependence of the number, depth and shape of the basins of the potential energy surface. We evaluate the total entropy of the system by thermodynamic integration from the ideal—noninteracting—gas state and the vibrational entropy approximating the basin free energy with the free energy of $6N-3$ harmonic oscillators. We evaluate the configurational part of the entropy as a difference between these two contributions. We study the connection between thermodynamical and dynamical properties of the system. We confirm that the temperature dependence of the configurational entropy and of the diffusion constant, as well as the inverse of the characteristic structural relaxation time, are strongly connected in supercooled states; we demonstrate that this connection is well represented by the Adam-Gibbs relation, stating a linear relation between $\log D$ and the quantity ${1/TS}_c.$ This relation is found to hold both above and below the critical temperature $T_c$—as previously found in the case of silica—supporting the hypothesis that a connection exists between the number of basins and the connectivity properties of the potential energy surface.