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SMASS = -3 | -2 | -1 | 0 |
||in VASP.4.4 and higher (micro canonical ensemble)
||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)
- 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
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 SMASS0 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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