Potential-tuning molecular dynamics studies of fusion, and the question of ideal glassformers: (I) The Gay-Berne model

Abstract

The ability of some liquids to vitrify during supercooling is usually seen as a consequence of the rates of crystal nucleation (and/or crystal growth) becoming small- thus a matter of kinetics. However there is evidence, dating back to the empirics of coal briquetting for maximum trucking efficiency, that ellipsoids pack efficiently when disordered. Noting that key studies of non-spherical object packing have never been followed from hard ellipsoids or spherocylinders (diatomics excepted) into the world of molecules with attractive forces, we have made a molecular dynamics MD study of crystal melting and glass formation on the Gay- Berne (G-B) model of ellipsoidal objects across the aspect ratio range of the hard ellipsoid studies. Here we report that, in the aspect ratio range of maximum ellipsoid packing efficiency, various G-B crystalline states, that cannot be obtained directly from the liquid, disorder spontaneously near 0 K and transform to liquids without any detectable enthalpy of fusion. Without claiming to have proved the existence of single component examples, we use the present observations, together with our knowledge of non-ideal mixing effects, to discuss the probable existence of "ideal glassformers" - single or multicomponent liquids that vitrify before ever becoming metastable with respect to crystals. The existence of crystal-free routes to the glassy state removes any precrystalline fluctuation perspective from the "glass problem". Unexpectedly we find that liquids with aspect ratios in the "crystallophobic" range also behave in an unusual (non-hysteritic) way during temperature cycling through the glass transition. We link this to the highly volume fraction-sensitive ("fragile") behavior observed in recent hard dumbbell studies at similar length/diameter ratios.