[Pw_forum] Strange behavior of phonon calculation

[Dal Corso Andrea]

On Sun, 2007-07-08 at 20:44 +0530, Srijan Kumar Saha wrote:
> 
> 
>    Dear pwscf users,
> 
>    My  calculated gamma phonon frequency for Si (spin-less)  is
>     = 510 cm-1    When  I switch on the spin  tag to specify the spin
> polarized
>    calculation for   the same sample, then I got   corresponding
>    gamma phonon frequency = 214 cm-1  
>    while all the other input parameters are exactly same.
> 
>    
>    Paolo wrote: 
>    please provide a job demostrating such behaviour
> 
>    Dear Paolo, 
>    I'm also sending the following input file what is demostrating such
> behaviour
>    by attachment.
>    
>  **************************************************************************************
>    #!/bin/sh
>  executable_dir=./
>  tmp_dir=./tmp
>  pseudo_dir=./pseudo
> cat > si.scf.in << EOF
>  &control
>     calculation='scf',
>     restart_mode='from_scratch',
>     prefix='si'
>     pseudo_dir = '$pseudo_dir/',
>     outdir='$tmp_dir/'
>  /
>  &system
>     ibrav = 2, celldm(1) =10.20, nat=  2, ntyp= 1,
>     ecutwfc = 18.0, nelec  =   8.00, nbnd =10
>     nspin=2, tot_magnetization=0,

The phonon code does not work with constrained magnetization. Nobody
programmed this part so far. Actually in this example the flag
tot_magnetization=0 is not needed because Si is non magnetic.
If you remove the flag the phonon code should work also with nspin=2.

Hope this helps,

Andrea




>     occupations='smearing', smearing='fd', degauss=0.005,
> /
>  &electrons
>     mixing_beta = 0.7
>     conv_thr =  1.0d-8
>  /
> ATOMIC_SPECIES
>  Si  28.086  Si.vbc.UPF
> ATOMIC_POSITIONS
>  Si 0.00 0.00 0.00
>  Si 0.25 0.25 0.25
> K_POINTS
>   10
>    0.1250000  0.1250000  0.1250000   1.00
>    0.1250000  0.1250000  0.3750000   3.00
>    0.1250000  0.1250000  0.6250000   3.00
>    0.1250000  0.1250000  0.8750000   3.00
>    0.1250000  0.3750000  0.3750000   3.00
>    0.1250000  0.3750000  0.6250000   6.00
>    0.1250000  0.3750000  0.8750000   6.00
>    0.1250000  0.6250000  0.6250000   3.00
>    0.3750000  0.3750000  0.3750000   1.00
>    0.3750000  0.3750000  0.6250000   3.00
> EOF
> $ECHO "  running the scf calculation...\c"
> $executable_dir/pw.x  <  si.scf.in > si.scf.out
> $ECHO " done"
> 
> # phonon calculation at Gamma
> cat > si.phG.in << EOF
> phonons of Si at Gamma
>  &inputph
>   tr2_ph=1.0d-14,
>   prefix='si',
>   amass(1)=28.08,
>   outdir='$tmp_dir/',
>   fildyn='si.dynG',
>  /
> 0.0 0.0 0.0
> EOF
> $ECHO "  running the phonon calculation at Gamma...\c"
> $executable_dir/ph.x < si.phG.in > si.phG.out
> $ECHO " done"
> 
> 
> ********************************************************************************
>   To run the spin less calculation I just remove tot_magnetization
> tag 
>   and reset nspin=1 and I get correct Gamma point
>   phonon frequency (510 cm-1) whereas above input file for spin
> calculation
>   will give us a completely wrong result (214 cm-1).
>  
>   Looking forward to your valuable suggestions.
>  
>   With best regards,
>   Srijan
-- 
Andrea Dal Corso                    Tel. 0039-040-3787428
SISSA, Via Beirut 2/4               Fax. 0039-040-3787528
34014 Trieste (Italy)               e-mail: dalcorso at sissa.it