[QE-users] Regarding phonon calculations

[Mona Asadinamin]

Dear Mohaddeseh:

You are right, it means IR and Raman active simultaneously.

Regarding the scf threshold, increasing it results in more accuracy and normally it should be very tight, something like 1.d-15 or even less. It definitely does not turn frequencies from positive to negative. It helps to bring the system out of unstable point in the potential energy surface that causes negative frequencies.

Best regards;

Mona Asadi Namin
Graduate student
Center for simulational physics
University of Georgia
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Email:mona.a at uga.ed<mailto:mona.a at uga.edu>u<mailto:mona.a at uga.edu>
Phone: +1-215-906-23-92


________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Mohad Abbasnejad <mohaddeseh.abbasnejad at gmail.com>
Sent: Wednesday, August 12, 2020 8:16 AM
To: users at lists.quantum-espresso.org <users at lists.quantum-espresso.org>
Subject: [QE-users] Regarding phonon calculations

[EXTERNAL SENDER - PROCEED CAUTIOUSLY]

Dear QE users,

Hello
I hope everybody is well.
I have two questions regarding phonon calculations.
1- I have calculated the frequency for special q point in my structure shown as follow:

     Mode symmetry, C_3v (3m)   point group:

     freq (  1 -  2) =        -59.6  [cm-1]   --> E    L_3        I+R
     freq (  3 -  3) =        -48.4  [cm-1]   --> A_1  L_1        I+R
     freq (  4 -  5) =        162.4  [cm-1]   --> E    L_3        I+R
     freq (  6 -  6) =        227.4  [cm-1]   --> A_1  L_1        I+R
     freq (  7 -  7) =        250.8  [cm-1]   --> A_2  L_2
     freq (  8 -  8) =        295.5  [cm-1]   --> A_2  L_2
     freq (  9 - 10) =        299.4  [cm-1]   --> E    L_3        I+R
     freq ( 11 - 12) =        308.1  [cm-1]   --> E    L_3        I+R
     freq ( 13 - 14) =        382.0  [cm-1]   --> E    L_3        I+R
     freq ( 15 - 16) =        435.0  [cm-1]   --> E    L_3        I+R
     freq ( 17 - 17) =        456.0  [cm-1]   --> A_1  L_1        I+R
     freq ( 18 - 18) =        513.6  [cm-1]   --> A_2  L_2
     freq ( 19 - 20) =        521.9  [cm-1]   --> E    L_3        I+R
     freq ( 21 - 21) =        571.4  [cm-1]   --> A_1  L_1        I+R
     freq ( 22 - 23) =        578.0  [cm-1]   --> E    L_3        I+R
     freq ( 24 - 24) =        614.8  [cm-1]   --> A_1  L_1        I+R
     freq ( 25 - 25) =        620.5  [cm-1]   --> A_2  L_2
     freq ( 26 - 27) =        635.4  [cm-1]   --> E    L_3        I+R
     freq ( 28 - 29) =        651.0  [cm-1]   --> E    L_3        I+R
     freq ( 30 - 30) =        653.0  [cm-1]   --> A_2  L_2

I am wondering if somebody can tell me the meaning of I+R modes.
Does it mean Raman and infrared are active simultaneously for this mode?

2- I am wondering if increasing the threshold for self-consistency may influence the frequencies significantly such that it causes the frequency value from positive to negative. Is the default of the program reasonable?

Any help would be appreciated.

Bests
Mohaddeseh
--
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Mohaddeseh Abbasnejad,
Assistant Professor of Physics,
Faculty of Physics,
Shahid Bahonar University of Kerman,
Kerman, Iran
P.O. Box 76169-133
Tel: +98 34 31322199
Fax: +98 34 33257434
Cellphone: +98 917 731 7514
E-Mail:     mohaddeseh.abbasnejad at gmail.com<mailto:mohaddeseh.abbasnejad at gmail.com>
Website:  academicstaff.uk.ac.ir/moabbasnejad<http://academicstaff.uk.ac.ir/moabbasnejad>
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