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Recent molecular dynamics (MD) simulations of liquid silica, using the``BKS'' model [Van Beest, Kramer and van Santen, Phys. Rev. Lett. {\bf 64},1955 (1990)], have demonstrated that the liquid undergoes a dynamical crossoverfrom super-Arrhenius, or ``fragile'' behavior, to Arrhenius, or ``strong''behavior, as temperature $T$ is decreased. From extensive MD simulations, weshow that this fragile-to-strong crossover (FSC) can be connected to changes inthe properties of the potential energy landscape, or surface (PES), of theliquid. To achieve this, we use thermodynamic integration to evaluate theabsolute free energy of the liquid over a wide range of density and $T$. We usethis free energy data, along with the concept of ``inherent structures'' of thePES, to evaluate the absolute configurational entropy $S_c$ of the liquid. Wefind that the temperature dependence of the diffusion coefficient and of $S_c$are consistent with the prediction of Adam and Gibbs, including in the regionwhere we observe the FSC to occur. We find that the FSC is related to a changein the properties of the PES explored by the liquid, specifically an inflectionin the $T$ dependence of the average inherent structure energy. In addition, wefind that the high $T$ behavior of $S_c$ suggests that the liquid entropy mightapproach zero at finite $T$, behavior associated with the so-called Kauzmannparadox. However, we find that the change in the PES that underlies the FSC isassociated with a change in the $T$ dependence of $S_c$ that elucidates how theKauzmann paradox is avoided in this system. Finally, we also explore therelation of the observed PES changes to the recently discussed possibility thatBKS silica exhibits a liquid-liquid phase transition, a behavior that has beenproposed to underlie the observed polyamorphism of amorphous solid silica.Comment: 14 pages, 18 figure

Free energy and configurational entropy of liquid silica: Fragile-to-strong crossover and polyamorphism