[Pw_forum] Negative frequency in phonon calculation

[Sanjeev Gupta]

The reason of negative frequencies are :
1. Look you pseudopotential perhaps use another and see.
2. Your structure is not well optimized.

and many...

wishes
sanjeev


On Sun, Dec 13, 2015 at 8:02 PM, Barnali Bhattacharya <
barnalidgbhatt at gmail.com> wrote:

> Dear Sir/Madam
>
>
>
> I am a quantum espresso user and interested in phonon calculation of BN
> doped two dimensional system. For these first of all I have checked the ecut
> off convergence and taking this ecutwfc value checked the K-point
> convergence. After taking this ecutwfc and k-point value, I have optimized
> the structure with ‘VC-relax option’ with convergence criteria
>
> “etot_conv_thr = 1.0D-16,
>
>  forc_conv_thr = 1.0D-16,”.
>
>
>
> Taking the relaxed position and cell parameter I have checked the suitable
> lattice constant for which energy is minimum by using the script…
>
>
>
> ……………………………………………
>
> #!/bin/sh
>
> …………….
>
> …………
>
> …………
>
> for a in `seq -w 0.995 0.001 1.005 `
>
> do
>
> …………………………..
>
> …………
>
> ……………….
>
> cat > $MOL.$a.in << EOF
>
> &control
>
>     calculation='relax',
>
>     restart_mode='from_scratch',
>
> ………………….
>
> ……………….
>
> ……………..
>
> …………………
>
> /
>
> CELL_PARAMETERS alat
>
> `echo $a*12.901074275 | bc -l` 0.000000000  0.000000000
>
> `echo $a*6.45053713| bc -l`  `echo $a*11.1726581| bc -l` 0.00000
>
>  0.000000000   0.000000000  29.000000000
>
> ATOMIC_SPECIES
>
> C 12.0107 C.pz-n-rrkjus_psl.0.1.UPF
>
> ATOMIC_POSITIONS (crystal)
>
> .............................
>
> ……………………………………..
>
> …………………………………..
>
> ………………………………
>
> …………………………………
>
> …………………………………..
>
> ……………………………………………………………………………………………………………………………………………………………………
>
>
>
> Then taking the lattice constant for which energy is minimum and the
> atomic position we have calculated phonon dispersion curve, but found
> negative frequency.
>
>
>
> I am confused is there any mistake in my procedure.
>
>
>
> Could anyone please guide me how this negative frequency will be
> disappear? Any help will be thankfully acknowledged.
>
>
>
> Thanking you.
>
>
>
> Sincerely
>
> Barnali Bhattacharya
>
> Ph.D student
>
> Department of physics
>
> Assam University
>
>
>
>
>
>
>
>
>
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-- 
With Best Regards,

------------------------------------
Dr. Sanjeev Kumar Gupta
Fulbright Post-Doctoral Scholar
Dept. of Physics
Michigan Technological University
1400 Townsend Drive, Houghton
MI 49931, USA
------------------------------------
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