[Pw_forum] Negative frequency in phonon calculation

[Barnali Bhattacharya]

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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