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
