[Pw_forum] My vague understanding of internal degrees of freedom.

[Hongsheng Zhao]

Hi all,

I've learned that the single-point energy calculation 
(calculation='scf') can be used for the following purpose:

---------
It can be used to calculate an equation of state (i.e. a pressure-volume 
and/or energy-volume dependence) for high-symmetry systems with no 
internal degrees of freedom, as long as the Stress property is specified.
---------

But I've some puzzles on the meaning of internal degrees of freedom. 
Say, for a bcc structure, does it has internal degrees of freedom or 
not?   Furthermore, in the general case, how can I know whether a system 
has the internal degrees of freedom or not?

ps.  According to the paper J. Phys.: Condens. Matter 21 395502, the 
internal degrees of freedom, also can be called the microscopic degrees 
of freedom, is the atomic coordinates; and the macroscopic degrees of 
freedom are the shape and size of the unit
cell.
-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China