[Pw_forum] My vague understanding of internal degrees of freedom

[Hongsheng Zhao]

On 07/22/2011 07:46 PM, RCP wrote:
>
>   Hello,
>
>
> Hongsheng Zhao wrote:
>> On 07/22/2011 03:49 PM, xiaochuan Ge wrote:
>>
> ...
>> Based on the above description, let we back to my question again: I
>> want know for a specific system, how we know whether there are "internal
>> degrees of freedom" or not.  How to judge it?
>>
>
>   By symmetry.
>   As you said, in order to calculate the elastic constants, one applies
> an (infinitesimal) uniform
>   strain. This will generally break the original symmetry of the cell.
> Now the question is: can one
>   assure that forces on atoms are zero for this new (lower) symmetry ?.
> If the answer is YES, then
>   do not worry about internal degrees of freedom. Contrarily, if it is
> NO, then you must account for
>   internal degrees of freedom.
>
>   Thus your best bet for a complex unit cell is that there are internal
> degrees of freedom, and the
>   difference among the two situations, namely, with/without, just means
> an energy difference
>   between the relaxed/unrelaxed atomic coordinates.

Thanks a lot, I've got it.

Regards
-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China