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A Computer Simulation Study of Supercooled Liquid and Amorphous-Solid Methanol

The constant-pressure molecular dynamics technique has been used to study the structural consequences of cooling liquid methanol. We employed a periodically replicated system of 250 molecules interacting with an intermolecular potential fitted to thermodynamic properties of the liquid. Plots of the enthalpy and volume against temperature exhibit a distinct change in slope around 160 K; a feature that is identified with the onset of structural arrest, occurring on the picosecond time-scale, in the supercooled liquid. The corresponding glass transition in real methanol occurs around 103 K. The main structural feature that accompanies the structural arrest is the appearance of a splitting in the primary peak of the radial distribution function for methyl groups. This observation contrasts strongly with the behaviour of atomic systems where such splittings arise only in the second peaks of distribution functions. The calculations are discussed in the light of recent neutron scattering data on glassy methanol.