[Pw_forum] ph.x - nat_todo option: 13 ions to be displaced, but 243 modes calculated

[Paolo Giannozzi]

On Sat, 2012-11-03 at 07:44 -0200, Antonio wrote:

> I'm running a phonon calculation (at gamma point) for a system containing 81 ions (243 modes), but allowing the displacement of only 13 ones (in order to check out this approximation for my system). According to the output file, there are 39 modes to be done. So, what do other frequencies shown mean?

I don't know much about the option you are using, but I think that it might be useful (maybe,
and with care) only if you want to compare the vibrational modes of an isoolated molecule 
with those of the molecule adsorbed on a surface. In principle, what you get from the
diagonalization of the partial dynamical matrix is meaningless. 

P.
-- 
Paolo Giannozzi, IOM-Democritos and University of Udine, Italy