TEBEG

TEBEG = [real]
Default: TEBEG = 0 

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

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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: VASP defines the temperature as

$T= \frac{1}{ k_B T 3 (N_{\rm ions}-N_{\rm constraints})} \sum\limits_{n}^{N_{\rm ions}} M_n | \vec v_n |^2.$

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.

• 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 also taken into account in the OUTCAR file. Hence the temperature output in the OUTCAR file is the actual simulation temperature. This means that the real simulation temperature is: T=TEBEG×N_ions/(N_ions-1). 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: TEBEG=T_requested×(N_ions-1)/N_ions.

• Andersen thermostat:

Same as for Nose-Hoover thermostat.

• Langevin thermostat:

As for the Nose-Hoover thermostat in this thermostat the number of degrees of freedom including constraines are already acounted for. The center of mass is also subtracted but the centre of mass is not conserved in this method since stochastic forces are acting on the atoms. Consequently, the temperature in the OUTCAR file has to be corrected to T=TEBEG×(N_ions-1)/N_ions

Related Tags and Sections

TEEND, IBRION, SMASS

Examples that use this tag

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