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

[Kan-Ju Lin]

Dr. Giannozzi,

Thank you for your response. The attachment is my ph.x output file. I use nat_todo to allow only atom #25 to displace. The output file indicates that only representation 30 and 39 are to be done (the irreps). #39 has two modes, and #30 has one. In the output, there are three negative and three positive frequencies. Is there a way to distinguish which three are the meaningful ones? 
 


Best,
Kan-Ju

On Mar 21, 2014, at 8:29 AM, Paolo Giannozzi <paolo.giannozzi at uniud.it> wrote:

> 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 
> 
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