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

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

VASP only writes the input value of the temperature 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 not taken into account in the OUTCAR file. 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.

• 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 not conserved in this method since stochastic forces are acting on the atoms. Hence the temperature output in the OUTCAR file is the actual simulation temperature.

• Andersen thermostat:

Same as for Langevin thermostat.

Related Tags and Sections

TEEND, IBRION, SMASS

Examples that use this tag

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