Dear all,
Thanks for your kind reply. I will try your suggestion.
Actually I am using a initial box size .After MD.veriable_cell
optimization I have got the following box size.
%block LatticeVectors
99.65551731800 0.00049203400 -0.00010718500
0.00049195700 99.65368231000 -0.00012343700
-0.00001368900 -0.00001574900 12.81571569100
%endblock LatticeVectors
I have used the this lattice vector and following parameter (Which is
included through PhononFC.fdf) for my force calculation.
NumberOfAtoms 144
MD.FCfirst = 49
MD.FClast =96
MD.TypeOfRun FC
MD.FCdispl 0.0400000000 Bohr
%block AtomicCoordinatesAndAtomicSpecies
...........
.........
%endblock AtomicCoordinatesAndAtomicSpecies
Thanking you.
_________________
Saikat Paik
IIT Kanpur
UP,India
>>
>> Dear Saikat
>> We agree with Roberto, MC=150 Ry is pretty low if you need to get good
>> phonon properties for graphene like materials.
>> Another delicate point would be the kpoint sampling of your system. I
>> did
>> not see the size of your box, so Kgridcutoff is meaningless.
>> Remember, good phonons need good forces, good forces need good energies!
>>
>
> Further, i would like to add a little to what Dr. Ricardo Faccio
> said ...........
> good (convereged) energies need good basis and good pseudopotentials (norm
> conserving in siesta).
>
> regards,
>
> Sonu Kumar
> Phd student, physics department, IITD.
>
> MeshCutoff and kpoint sampling is critical.
>> Regards
>> Ricardo
>>
>> El 15/10/2012 09:12, R.C.Pasianot escribió:
>>
>>
>>> Hi Saikat,
>>>
>>> Assuming your structure is stable, negative frequencies generally
>>> mean poor convergence of either, the optimized structure or the
>>> following FC run.
>>>
>>> I have no experience with CNT, though the parameters you're using
>>> seem pretty ok to me, but for one: it is "Meshcutoff" not "Messcutoff"
>>> as you type.
>>> In any case, 150 Ry might be on the low side (depending on box's
>>> size).
>>> It is a particularly critical parameter, check the manual for the
>>> "eggbox" effect.
>>> Also, you might want to check the *.FC file that Siesta outputs.
>>> Namely,
>>> the forces for + and - displacements should be about the same,
>>> particularly on the moving atom (file structure has been explained
>>> in a recent post to the list).
>>>
>>> Good luck,
>>>
>>> Roberto
>>>
>>>
>>> On Mon, 15 Oct 2012, [email protected] wrote:
>>>
>>> Dear siesta user,
>>>>
>>>> I am doing phonon calculation of carbon nanotube using siesta. But I
>>>> am
>>>> getting negative frequencies for more than half of the calculated
>>>> frequencies. I wanted to know reason for getting negative frequencies
>>>> of my calculation.
>>>>
>>>>
>>>> I have optimized the structure using
>>>>
>>>> MD.MaxForceTol 0.001 eV/Ang
>>>>
>>>>
>>>> And condition for scf are as follows
>>>>
>>>> DM.Tolerance 0.5d-4
>>>> DM.Require.Energy.Convergence true
>>>> DM.Energy.Tolerance .5d-4 eV
>>>>
>>>> My input file for type of run- FC ----
>>>>
>>>>
>>>>
>>>> SystemName Phonon calculation # Descriptive name of the
>>>> system
>>>> SystemLabel Phonon_fc # Short name for naming
>>>> files
>>>>
>>>> # Output options
>>>>
>>>> LongOutput false
>>>>
>>>> # Species and atoms
>>>> %block ChemicalSpeciesLabel
>>>> 1 6 C
>>>> %endblock ChemicalSpeciesLabel
>>>>
>>>> NumberOfSpecies 1
>>>> WriteKpoints true
>>>>
>>>>
>>>> PAO.Basis<basis.fdf
>>>>
>>>> LatticeConstant 1 Ang
>>>> Messcutoff 150 Ry
>>>> Kgridcutoff 10.0 Ang
>>>> xc.functional GGA
>>>> xc.authors PBE
>>>> DM.UseSaveDM true
>>>> MaxSCFIterations 500
>>>> DM.MixingWeight 0.25
>>>> DM.NumberPulay 4
>>>>
>>>> DM.Tolerance 0.4d-4
>>>> DM.Require.Energy.Convergence true
>>>> DM.Energy.Tolerance .5d-4 eV
>>>> #Atomic coordinates
>>>> %include PhononFC.fdf
>>>> AtomicCoordinatesFormat Fractional
>>>>
>>>> Thanking You.
>>>>
>>>>
>>>> __________________
>>>>
>>>> Saikat Paik
>>>> IIT Kanpur
>>>> UP,India
>>>>
>>>>
>>>
>> --
>> ------------------------------**------------------------------**
>> -------------
>> Dr. Ricardo Faccio
>> Prof. Adjunto de Física
>> Mail: Cryssmat-Lab., Cátedra de Física, DETEMA
>> Facultad de Química, Universidad de la República
>> Av. Gral. Flores 2124, C.C. 1157
>> C.P. 11800, Montevideo, Uruguay.
>> E-mail: [email protected]
>> Phone: (+598) 2924 9859
>> (+598) 2929 0648
>> Fax: (+598) 2924 1906
>> Web:
>> http://cryssmat.fq.edu.uy/**ricardo/ricardo.htm<http://cryssmat.fq.edu.uy/ricardo/ricardo.htm>
>>
>>
>
