TEBEG: Difference between revisions

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{{TAGDEF|TEBEG|[real]|0}}
{{TAGDEF|TEBEG|[real]|0}}


Description: {{TAG|TEBEG}} sets the start temperature for an ab-initio molecular dynamics run ({{TAG|IBRION}}=0) and other routines (e.g. {{TAG|Electron-phonon interactions from Monte-Carlo sampling}}).
Description: {{TAG|TEBEG}} sets the starting temperature (in K) for an ab-initio molecular dynamics run ({{TAG|IBRION}}=0) and other routines (e.g. {{TAG|Electron-phonon interactions from Monte-Carlo sampling}}).
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If no initial velocities are supplied on the {{FILE|POSCAR}} file, the velocities are set randomly according to a Maxwell-Boltzmann distribution at the initial temperature {{TAG|TEBEG}}. Velocities are only used for molecular dynamics ({{TAG|IBRION}}=0).
If no initial velocities are supplied on the {{FILE|POSCAR}} file, the velocities are set randomly according to a Maxwell-Boltzmann distribution at the initial temperature {{TAG|TEBEG}}. Velocities are only used for molecular dynamics ({{TAG|IBRION}}=0).


'''Mind''': VASP defines the temperature as  
'''Mind''': If {{TAG|MDALGO}}>0 is used VASP defines the temperature as  
:<math>
:<math>
T= \frac{1}{3 k_B T N_{\rm ions}} \sum_n M_n | \vec v_n |^2.
T= \frac{1}{ k_B T 3 (N_{\rm ions}-N_{\rm constraints})} \sum\limits_{n}^{N_{\rm ions}} M_n | \vec v_n |^2.
</math>
</math>


VASP only writes the input value of the temperature to the {{FILE|OUTCAR}} file).  
This temperature ist written to the {{FILE|OUTCAR}} file.  
Depending on the type of thermostat this temperature has to be rescaled to obtain the real simulation temperature.  
Depending on the type of thermostat this temperature has to be rescaled to obtain the real simulation temperature.  


*{{TAG|Nose-Hoover thermostat}}:
*{{TAG|Nose-Hoover thermostat}}:
In this thermostat the number of degrees of freedom including constraines are already acounted for in the potential energy term. In this this method the center of mass is conserved. This lowers the degrees of freedom by one which is not taken into account in the {{TAG|OUTCAR}} file. This means that the real simulation temperature is: T={{TAG|TEBEG}}&times;N<sub>ions</sub>/(N<sub>ions</sub>-1).
In this thermostat the number of degrees of freedom including constraines are already acounted for in the potential energy term. In this this method the center of mass is conserved. This lowers the degrees of freedom by one which is also taken into account in the {{TAG|OUTCAR}} file.  
Consequently, the temperature written by VASP (e.g. in the  is incorrect and has to be corrected in accordance with the above. Usually the effect is rather small and subtle, but one should correct the error if very precise results are required; in that case the temperature should be specified according to: {{TAG|TEBEG}}=T<sub>requested</sub>&times;(N<sub>ions</sub>-1)/N<sub>ions</sub>.
 
*{{TAG|Langevin thermostat}}:
As for the {{TAG|Nose-Hoover thermostat}} in this thermostat the number of degrees of freedom including constraines are already acounted for. The center of mass is not converged in this method since stochastic forces are acting on the atoms. Hence the temperature output in the {{TAG|OUTCAR}} file is the actual simulation temperature.


*{{TAG|Andersen thermostat}}:
*{{TAG|Andersen thermostat}}:
Same as for {{TAG|Langevin thermostat}}.
Same as for {{TAG|Nose-Hoover thermostat}}.


*{{TAG|Langevin thermostat}}:
As for the {{TAG|Nose-Hoover thermostat}} and {{TAG|Andersen thermostat}} in this thermostat the number of degrees of freedom including constraines are already acounted for. The center of mass is not conserved in this method, hence this method has 3 degrees of freedom more than the {{TAG|Nose-Hoover thermostat}} and {{TAG|Andersen thermostat}}.


== Related Tags and Sections ==
== Related tags and articles ==
{{TAG|TEEND}},
{{TAG|TEEND}},
{{TAG|IBRION}},
{{TAG|IBRION}},
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[[Category:INCAR]][[Category:Molecular Dynamics]]
[[Category:INCAR tag]][[Category:Molecular dynamics]]

Latest revision as of 12:22, 7 September 2022

TEBEG = [real]
Default: TEBEG = 0 

Description: TEBEG sets the starting temperature (in K) for an ab-initio molecular dynamics run (IBRION=0) and other routines (e.g. Electron-phonon interactions from Monte-Carlo sampling).


If no initial velocities are supplied on the POSCAR file, the velocities are set randomly according to a Maxwell-Boltzmann distribution at the initial temperature TEBEG. Velocities are only used for molecular dynamics (IBRION=0).

Mind: If MDALGO>0 is used VASP defines the temperature as

This temperature ist written to the OUTCAR file. Depending on the type of thermostat this temperature has to be rescaled to obtain the real simulation temperature.

In this thermostat the number of degrees of freedom including constraines are already acounted for in the potential energy term. In this this method the center of mass is conserved. This lowers the degrees of freedom by one which is also taken into account in the OUTCAR file.

Same as for Nose-Hoover thermostat.

As for the Nose-Hoover thermostat and Andersen thermostat in this thermostat the number of degrees of freedom including constraines are already acounted for. The center of mass is not conserved in this method, hence this method has 3 degrees of freedom more than the Nose-Hoover thermostat and Andersen thermostat.

Related tags and articles

TEEND, IBRION, SMASS

Examples that use this tag