Average Structure vs. Real Structure: Molecular Dynamics Studies of Silica
The microscopic structure of a crystal and thermal fluctuations of the atoms constituting the crystal are intimately connected with the macroscopic elastic properties including mechanical stability. In some cases, however, the picture is more complex than that which is drawn in text books on solid state physics. (i) The instantaneous microscopic structure can deviate in a non-Gaussian way from the average structure even when domain disorder and/or crystal defects are absent. Quasi harmonic approximations may then turn out to be meaningless. (ii) The crystal is subject to external pressures that are sufficiently large in order to render the definition of elastic constants non unique. These two points are discussed exemplarily in the context of the high-temperature and the high-pressure phases of quartz. In particular, it is discussed how to observe and how to classify non-Gaussian disorder in molecular dynamics (MD) simulations and how to evaluate mechanical stability of solids under pressure. Some details are given on the calculation of thermal, mechanical, and structural properties of solids, also for temperatures far below their Debye temperature.
KeywordsElastic Constant Radial Distribution Function Harmonic Approximation Simulation Cell Landau Theory
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