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Next: 7.28 NPACO and APACO-tag Up: 7 The INCAR File Previous: 7.26 TEBEGTEEND-tag

7.27 SMASS-tag



The default has been changed to -3 (i.e. micro canonical ensemble).

SMASS controls the velocities during an ab-initio molecular dynamics.

For SMASS=-3 a micro canonical ensemble is simulated. The calculated Hellmann-Feynman forces serve as an acceleration acting onto the ions. The total free energy (i.e. free electronic energy + Madelung energy of ions + kinetic energy of ions) is constant.
For SMASS=-2 the initial velocities are kept constant. This allows to calculate the energy for a set of different linear dependent positions (for instance frozen phonons, section 11.9, dimers with varying bond-length, section 11.5).

Mind: if SMASS=-2 the actual steps taken are POTIM*read velocities. To avoid ambiguities, set POTIM to 1 (also read section 5.7 for supplying initial velocities).

In this case the velocities are scaled each NBLOCK step (starting at the first step i.e. MOD(NSTEP,NBLOCK).EQ.1) to the temperature


where NSTEP is the current step (starting from 1). This allows a continuous increase or decrease of the kinetic energy. In the intermediate period a micro-canonical ensemble is simulated.

For SMASS>=0 a canonical ensemble is simulated using the algorithm of Nosé. The Nosé mass controls the frequency of the temperature oscillations during the simulation (see [1, 2, 3]. For SMASS=0 Nosé-mass corresponding to period of 40 time steps will be chosen. The Nosé-mass should be set so that the induced temperature fluctuation show approximately the same frequencies as the typical 'phonon'-frequencies for the specific system. For liquids something like 'phonon'-frequencies might be obtained from the spectrum of the velocity auto-correlation function. If the ionic-frequencies differ by an order of magnitude from the frequencies of the induce temperature fluctuations Nosé thermostat and ionic movement might decouple leading to a non canonical ensemble. The frequency of the approximate temperature fluctuations induced by the Nosé-thermostat can be found on the OUTCAR file.

next up previous contents
Next: 7.28 NPACO and APACO-tag Up: 7 The INCAR File Previous: 7.26 TEBEGTEEND-tag

Mon Mar 29 10:38:29 MEST 1999