When you have 67 negative frequencies out of 132, it is often that
the XC fucntional is not the same with the relaxed solution and with the
displaced position computed later. When you run SIESTA with FC do not forget
to specify again XC functional and authors.
Be careful with the warning, because it underlines this problem.
Once it took me a week to realize it.
Alexandre
"R.C.Pasianot" <[email protected]> a écrit :
Ciao Giacomo,
I know nothing about Born charges, but did FC calculations a number
of times: 67 negative frequencies out of 132 means your calculation
is garbage. FC calculations are quite delicate in the sense that you
need, 1st very well relaxed structures, and 2nd very well converged
forces in the FC calculation step.
At 1st glance, the only thing I wonder regarding your data file is
if those 400 Ry of MeshCutoff are large enough to eliminate the
egg box effect for a box having an edge of 40 Ang along z axis.
This might be affecting force precision while evaluating the FC
matrix. You might tell if something along these speculations is
going on by looking at the FC file itself; watch e.g. if for
opposite displacements you are getting opposite forces, check that
the largest force occurs on the atom being displaced, that the
total force for the whole bov is a good zero. etc.
Good luck,
Roberto
On Thu, 6 Sep 2012, Giacomo Giorgi wrote:
Dear All,
I am doing for the first times the calculation of Frequency
with SIESTA. My system is an asymmetric slab (2 bottom layers are
frozen) and on top of it I anchored an organic molecule.
In total 92 atoms. But freezing the first two layers I need to know
the frequencies of only 44 atoms, i.e. 44*3 =132 Eigenvectors.
Anyway, by checking both the .bands and the .vectors files I
observe that the frequencies are highly underestimated with respect
to the experimental ones (my last calculated frequency, the 132nd,
is at 1649.101 cm-1, while
experimentally a net visible peak is at 2900 cm-1)
Moreover the first 67 (!!) of 132 Eigenvectors are characterized by
a negative Frequency.
Below my Vibra .fdf
Any comment and hint is extremely appreciated.
Additionally, as previously posted (but no answer) even if I set
BornCharge True
in the fdf, no .BC file was generated thus no IR Spectrum... :(
Many thanks,
Giacomo
SystemName 101
SystemLabel 101
################# options for Vibra ####################
Eigenvectors .true.
SuperCell_1 0
SuperCell_2 0
SuperCell_3 0
BandLinesScale ReciprocalLatticeVectors
%block BandLines
1 0.000 0.000 0.000 \Gamma
%endblock BandLines
# Kpoints
KPointMPSampling F
%block PolarizationGrids
6 3 3
4 8 4
5 5 10
%endblock PolarizationGrids
BornCharge True
########################################################
MD.TypeOfRun FC # Type of dynamics:
MD.FCfirst 49
MD.FClast 92
MD.FCdispl 0.02 ang
NumberOfAtoms 44
NumberOfSpecies 4
%block ChemicalSpeciesLabel
1 22 Ti
2 8 O
3 6 C
4 1 H
%endblock ChemicalSpeciesLabel
PAO.BasisSize DZP
XC.functional LDA
XC.authors CA
ElectronicTemperature 1500.0 K
LatticeConstant 1. Ang
%block LatticeVectors
7.569 0 0
0 10.2394 0
0 0 40
%endblock LatticeVectors
MeshCutoff 400.0 Ry
MaxSCFIterations 200
DM.MixingWeight 0.008
DM.NumberPulay 5
DM.Tolerance 1.d-4
ParallelOverK .true.
SolutionMethod diagon
WriteDM .true.
DM.UseSaveDM .true.
PAO.EnergyShift 0.01 Ry
MD.MaxForceTol 0.01 eV/Ang
WriteForces true
WriteCoorCerius true
WriteCoorXmol true
WriteMullikenPop 1
WriteMDXmol true
UseSaveData .true.
%block GeometryConstraints
position from 1 to 48 # 2 layers
%endblock GeometryConstraints
AtomicCoordinatesFormat NotScaledCartesianAng
%block AtomicCoordinatesAndAtomicSpecies
....
....
%endblock AtomicCoordinatesAndAtomicSpecies
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