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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.

## 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).