[Pw_forum] 3 additional frequencies (6 total) when only displace one atom in the PH.x calculation

[Paolo Giannozzi]

On Tue, 2014-03-18 at 16:57 +0000, Kan-Ju Lin wrote:

> To facilitate the frequency calculation, I used nat_todo = 1 to only
> allowing the helium to be displaced [...]

> My questions are:
> 1. Why are there 3 additional frequencies? Shouldn’t there only be 3
> (omega 73~75) due to the DoF of one atom?

nat_todo means that the calculation is performed only for irreducible
representations containing a displacement of the chosen atoms(s). 
In the output of the phonon code, you should see how many irreps are
used and their degeneracy. If there are 6 frequencies, there should 
be a total of 6 irreps. What you get as frequencies may not have any
special physical meaning, with the exception of some special cases
(e.g. molecule on a surface with well-separated modes)

Paolo

> 2. Why are the three additional frequencies negative? Shouldn’t there
> be one negative frequency for the saddle point, and 2 positive
> frequencies? and no negative frequency for the energy minimum?
> 
> 
> Please let me know if I understand this incorrectly, or I am missing
> some settings.
> 
> 
> 
> 
> --
> Kan-Ju Lin, Postdoctoral Associate
> MIT Materials Science and Engineering
> Office 4-053 / kjlin at mit.edu
> 
> 
> 
> 
> 
> 
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-- 
 Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
 Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
 Phone +39-0432-558216, fax +39-0432-558222