[Pw_forum] Fixing atoms in vc-relax

[Maxim Popov]

Dear Sridhar Sadasivam,

Fixing atoms by specifying trailing zeros leads to fixing atomic
coordinates within the cell, i.e., their fractional coordinates.
When doing 'relax' type of calculation, keeping fractional coordinates
unchanged is equivalent to keeping Cartesian coordinates the same, thus
distances between the fixed atoms are preserved. Situation becomes
different in case of 'vc-relax', now the cell parameters may change, and
even if the fractional coordinates of some atoms remain unchanged, their
Cartesian coordinates are likely to change as well as the distances between
the 'fixed' atoms.

HTH,
Maxim.


2014-03-03 10:14 GMT+01:00 Sridhar Sadasivam <sridhu88 at gmail.com>:

> Hello,
>
> I am trying to fix some atoms in a vc-relax simulation, i.e., I want the
> relative positions of these atoms to remain the same during vc-relaxation.
> I specified 0 0 0 next to the atomic positions of these atoms. However
> their relative positions do change. I have tried giving the coordinates of
> these atoms in both crystal and angstrom units with no luck.
>
> Their relative positions do remain constant in a relax simulation but not
> in a vc-relax simulation. Does anyone know what might be the problem?
>
> Thanks,
> Sridhar
>  Purdue University,
> West Lafayette, IN 47906
>
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-- 
Best regards,
Dr. Maxim N. Popov
Materials center Leoben (MCL), Leoben, Austria.
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