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7.27 SMASS-tag

 

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

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

-3
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.
-2
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).

-1
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

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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.

>=0
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

MASTER USER VASP
Mon Mar 29 10:38:29 MEST 1999