A multiscale model of phase-change memory is proposed. A phase transition is self-consistently simulated on three different time-space levels. On the first level, we use ab initio quantum molecular dynamics calculations with consideration of a temperature distribution. On the second level, the time-dependent evolution of the electronic density is simulated on the basis of the reduced Ehrenfest molecular dynamics near the line of the phase transition of the second kind. On the third level, we use the heat conduction equation in continuous media to calculate a new temperature distribution. Our calculations show the appearance of a graphitic layer structure from an amorphous state under the influence of temperature effects. The electronic density evolution leads to a localization in space of the electric conductivity. A space-localized heat source causes the thermal instability and, hence, maintains the structure. Such a behavior could explain the appearance of the S-shaped volt-ampere characteristic in a conducting nanodot during experiments. The IBM BlueGene/P supercomputer installed at the Faculty of Computational Mathematics and Cybernetics of the Lomonosov Moscow State University was used.

Keywords: multiscale quantum molecular dynamics codes, phase transition in amorphous carbon, phase change memory, nanotechnology, supercomputer IBM BlueGene/P.

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