[Pw_forum] My vague understanding of internal degrees of freedom

[RCP]

 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.

 HTH

 Roberto