SMASS: Difference between revisions

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* {{TAG|SMASS}}=-3
* {{TAG|SMASS}}=-3
:For {{TAG|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 {{TAG|SMASS}}=-3 a micro canonical ensemble ({{TAG|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 {{TAG|MDALGO}}=0 (not with {{TAG|MDALGO}}=2) and is hence rather considered as obsolete. To calculate an {{TAG|NVE_ensemble}} we recommend the user to use {{TAG|MDALGO}}=1 and {{TAG|ANDERSEN_PROB}}=0.0.


* {{TAG|SMASS}}=-2
* {{TAG|SMASS}}=-2
:For {{TAG|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).
:For {{TAG|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 {{TAG|SMASS}}=-2 the actual steps taken are {{TAG|POTIM}}×read velocities. To avoid ambiguities, set {{TAG|POTIM}}=1 (read the article on the {{FILE|POSCAR}} file to see how to supply the initial velocities).
:'''Mind''': if {{TAG|SMASS}}=-2 the actual steps taken are {{TAG|POTIM}}×(velocities-read-from-the-{{FILE|POSCAR}}-file). To avoid ambiguities, set {{TAG|POTIM}}=1.


* {{TAG|SMASS}}=-1
* {{TAG|SMASS}}=-1
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* {{TAG|SMASS}}≥0
* {{TAG|SMASS}}≥0
:For {{TAG|SMASS}}&ge;0, a canonical ensemble is simulated using the algorithm of Nosé. The Nosé mass controls the frequency of the temperature oscillations during the simulation.<ref name="nose:jcp:84"/><ref name="nose:ptps:91"/><ref name="bylander:prb:92"/> For {{TAG|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 {{FILE|OUTCAR}} file.
:For {{TAG|SMASS}}&ge;0, a canonical ensemble is simulated using the algorithm of Nosé. The Nosé mass controls the frequency of the temperature oscillations during the simulation.{{cite|nose:jcp:1984}}{{cite|nose:ptp:1991}}{{cite|bylander:prb:1992}} For {{TAG|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 {{FILE|OUTCAR}} file.


== Related Tags and Sections ==
== Related Tags and Sections ==
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{{TAG|TEBEG}},
{{TAG|TEBEG}},
{{TAG|TEEND}}
{{TAG|TEEND}}
{{sc|SMASS|Examples|Examples that use this tag}}


== References ==
== References ==
<references>
<references/>
<ref name="nose:jcp:84">[http://dx.doi.org/10.1063/1.447334 S. Nos&eacute;, J. Chem. Phys. 81, 511 (1984).]</ref>
 
<ref name="nose:ptps:91">S. Nos&eacute;, Prog. Theor. Phys. Suppl. 103, 1 (1991).</ref>
<noinclude>
<ref name="bylander:prb:92">[http://link.aps.org/doi/10.1103/PhysRevB.46.13756 D. M. Bylander, L. Kleinman, Phys. Rev. B 46, 13756 (1992).]</ref>
</references>
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[[The_VASP_Manual|Contents]]


[[Category:INCAR]][[Category:Dynamics]]
[[Category:INCAR]][[Category:Molecular Dynamics]][[Category:Thermostats]]

Revision as of 10:22, 27 June 2019

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

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


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.
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.
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.
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