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

Kan-Ju Lin kjlin at mit.edu
Tue Mar 18 17:57:47 CET 2014

```Hi all,

I am using QE 5.0.3.

I am trying to test if a saddle point from the neb.x calculation is a real saddle point. I am doing this by looking for the vibrational frequency: If a state is a saddle point, it has only one imaginary (negative) frequency. If a state is an energy minimum, it has no imaginary frequency.

My system consists of a crystal structure and a helium impurity in it. The crystal structures at energy minimum and the saddle point are almost identical, only the helium atom is displaced. To facilitate the frequency calculation, I used nat_todo = 1 to only allowing the helium to be displaced:

phonons of initial state at Gamma
&inputph
nat_todo = 1
tr2_ph=1.0d-14,
prefix = "te",
epsil=.true.,
/
0.0 0.0 0.0
25

The last line, 25, indicates only the 25th atom (helium) is displaced during the ph.x calculation. Because there are 25 atoms total in the system, there should be 75 modes. The complete ph.x outputs are attached. The energy minimum state has the frequencies:

omega( 1) =      -2.080553 [THz] =     -69.399779 [cm-1]
omega( 2) =      -2.080553 [THz] =     -69.399779 [cm-1]
omega( 3) =      -2.080553 [THz] =     -69.399779 [cm-1]
……….
……….
omega(73) =       4.621913 [THz] =     154.170425 [cm-1]
omega(74) =       4.621913 [THz] =     154.170425 [cm-1]
omega(75) =       4.621913 [THz] =     154.170425 [cm-1]

with omega(4~72) all equal to zero. For the saddle point one:

omega( 1) =      -4.316979 [THz] =    -143.998928 [cm-1]
omega( 2) =      -2.270462 [THz] =     -75.734475 [cm-1]
omega( 3) =      -2.270462 [THz] =     -75.734475 [cm-1]
……….
……….
omega(73) =       2.168375 [THz] =      72.329209 [cm-1]
omega(74) =       9.304582 [THz] =     310.367434 [cm-1]
omega(75) =       9.304582 [THz] =     310.367434 [cm-1]

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?
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<mailto:kjlin at mit.edu>

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