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SMASS = -3 | -2 | -1 | 0 |     
SMASS = -3 in VASP.4.4 and higher (micro canonical ensemble)
  = 0 releases older that VASP.4.4

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

For SMASS=-3 a micro canonical ensemble is simulated (constant energy molecular dynamics). 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 conserved.
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 9.9, dimers with varying bond-length, section 9.6).

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

$\displaystyle TEMP={\tt TEBEG}+({\tt TEEND}-{\tt TEBEG})*NSTEP/{\tt NSW}

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 such 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 induced 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 is written to the OUTCAR file.

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