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SMASS

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SMASS = -3 | -2 | -1 | [real] ≥ 0
Default: SMASS = -3 

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


  • SMASS=-3
For SMASS=-3 a micro canonical ensemble (NVE 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. Mind: This option only works with MDALGO=0 (not with MDALGO=2) and is hence rather considered as obsolete. To calculate an NVE_ensemble we recommend the user to use MDALGO=1 and ANDERSEN_PROB=0.0.
  • SMASS=-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, or dimers with varying bond-length).
Mind: if SMASS=-2 the actual steps taken are POTIM×(velocities-read-from-the-POSCAR-file). To avoid ambiguities, set POTIM=1.
  • SMASS=-1
In this case the velocities are scaled each NBLOCK step (starting at the first step i.e. MOD(NSTEP,NBLOCK)=1) to the temperature: T=TEBEG+(TEEND-TEBEG)×NSTEP/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.
  • SMASS≥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.[1][2][3] For SMASS=0, a 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.

Related Tags and Sections

IBRION, POTIM, NBLOCK, TEBEG, TEEND

Examples that use this tag

References

  1. S. Nosé, J. Chem. Phys. 81, 511 (1984).
  2. [ S. Nosé, Prog. Theor. Phys. Suppl. 103, 1 (1991).]
  3. D. M. Bylander and L. Kleinman, Phys. Rev. B 46, 13756 (1992).