Hi Mousumi,
your zone-center acoustic frequency is not close enough to zero;
this is an indication of bad, or badly converged, forces.
(However you already get right sign and order of magnitude,
that's not always automatically the case :-)
Some considerations:
1. Your basis (SZ) is probably too small for a good phonon calculation,
2. your k-mesh (7*7*5) may be not fine enough.
3. Check if your MeshCutoff is sufficient to get small enough sum
of forces over all atoms in X,Y,Z directions independently
(200 Ry is not unreasonably low, but still...)
Try first to get the bulk modulus in diamond right, this is not so
lengthy calculation, so you can easily try different cutoffs.
With a good bulk modulus, it's a fair chance that you'll get the phonons
as well.
Good luck,
Andrei
+-- Dr. habil. Andrei Postnikov - Tel. +49-541-969.2377 -- Fax .2351 ---+
| Institut fuer Festkoerperforschung - FZ Juelich, D-52425 Juelich, Germany |
+-- [EMAIL PROTECTED] - http://www.home.uni-osnabrueck.de/apostnik/ --+
On Sat, 21 May 2005, Mousumi Upadhyay Kahaly wrote:
| Hi everybody,
|
| I am trying to relax then calculate the phonon frequencies of
| different carbon systems using siesta fcbuild vibra.
|
| For diamond structure- at Gamma point:
| my results- phonon freq= 0.3, 2042 cm-1. (only Gamma point calculation)
| phonon freq= 0.50, 2163 cm-1 (Gamma to L-direction, 20
| points),
| while expt result - 1333 cm-1.
|
| For graphite structure, at Gamma point,
| my results- phonon freq.= 0.0, 1121.27 2305.7 cm-1, (only Gamma point
| calculation)
|
| while expt result- 0.0, 900 1600 cm-1.
|
| Can anybody please suggest me, why are these discrepancies in
| phonon frequencies? What should I do now to get rid of this
| huge error?
|
| Here is my sample relaxation input file(that creats .FC file,
| used to get phonon freq. through vibrator) for graphite --
|
| best regards, mousumi
|
| ***
|
| SystemName Carbon in graphite # Descriptive name of the system
| SystemLabel graphite # Short name for naming files
|
| NumberOfAtoms 4# Number of atoms
| NumberOfSpecies 1# Number of species
|
| %block Chemical_Species_Label
| 16C
| %endblock Chemical_Species_Label
|
| PAO.BasisSize SZ
|
| # Lattice, coordinates, k-sampling
|
| %block kgrid_Monkhorst_Pack
| 7 0 00.0
| 0 7 00.0
| 0 0 50.0
| %endblock kgrid_Monkhorst_Pack
|
|
| AtomicCoordinatesFormat Fractional # Format for coordinates
| AtomicCoorFormatOut Ang
|
|
| %block AtomicCoordinatesAndAtomicSpecies
|0.3334326744 0.3334326744 0.00 1
|0.6668653488 0.6668653488 0.00 1
|0.00 0.00 0.50 1
|0.3334326744 0.3334326744 0.50 1
| %endblock AtomicCoordinatesAndAtomicSpecies
|
| %block LatticeVectors
| 1. 0. 0.00
| 0.5000 0.86602540378443864676 0.00
| 0.000.00 2.120
| %endblock LatticeVectors
|
| LatticeConstant2.510Ang
|
|
|
| #kgrid_cutoff7. Ang
|
| XC.functional LDA # Exchange-correlation functional type
| XC.authors CA # Particular parametrization of xc func
| SpinPolarized .false. # Spin unpolarized calculation
| MeshCutoff 200. Ry # Equivalent planewave cutoff for the
| grid
| MaxSCFIterations100 # Maximum number of SCF iterations per
| step
| DM.MixingWeight 0.3 # New DM amount for next SCF cycle
| DM.Tolerance1.d-4 # Tolerance in maximum difference
| # between input and output DM
| DM.NumberPulay 3 # Number of SCF steps between pulay
| mixing
|
| # Eigenvalue problem: order-N or diagonalization
|
| SolutionMethod diagon # OrderN or Diagon
| ElectronicTemperature 5 K# Temp. for Fermi smearing
|
| # Molecular dynamics and relaxations
|
| MD.TypeOfRunFC # Type of dynamics:
| #ElectronicTemperature 30 K# Temp. for Fermi smearing
| #MD.NumCGsteps 40
|
| # Output options
|
| WriteCoorInitial
| WriteCoorStep
| WriteForces
| WriteKpoints.true.
| WriteEigenvalues.false.
| WriteKbands .true.
| WriteBands .true.
| WriteMullikenPop1# Write Mulliken Population Analysis
| WriteCoorXmol .false.
| WriteMDCoorXmol .false.
| WriteMDhistory .false.
| WriteCoorXmol .false.
|
| # Options for saving/reading information
|
| DM.UseSaveDM
