> > 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> > >
