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

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

But, because the center of mass is conserved, there are only 3(N_ions-1) degrees of freedom (the sum of all velocities is zero, if a random initialization is chosen). 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 OUTCAR file) 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.

Related Tags and Sections

TEEND, IBRION, SMASS

Examples that use this tag

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