> Dear siesta users, > As we konw that there should be no negative value in phonon > spectrum, but when I tried to draw the phonon spectrum of > bilayer-graphene, there are always some negative intervals. I'am > very confused about this.
Dear Liu, such kind of "noise" in the vicinity of Gamma appears when the force constants are not enough converged in the real space (with distance to neighbors). You use the supercell (3 3 0); will (4 4 0) be affordable? However! - the supercell size will affect just the vicinity of Gamma, but NOT Gamma itself - which ought to be good even in (0 0 0) supercell. I cannot judge from your plot whether you have an imaginary frequency also at Gamma, or it is just traced so through many points. If you have a problem at Gamma, then you should fix it first, staying with a (0 0 0) supercell. It must be due to insufficient MeshCutoff (170 Ry you use is not that high). Best regards Andrei Postnikov > My program attached. > > Best regards > Liu > _____________________________________________________________- > > NumberOfSpecies 1 > > > > %block chemicalspecieslabel > > 1 6 C > > %endblock chemicalspecieslabel > > > > PAO.BasisSize DZP > > > > MeshCutoff 170.0 Ry > > DM.MixingWeight 0.001 /0.01/0.1 > > DM.NumberPulay 3 > > DM.Tolerance 1.0d-5 > > %include FC.fdf > > NumberOfAtoms 4 > > LatticeConstant 2.45951215 Ang > > > > %block LatticeVectors > > 0.8660254 0.5 0.0 > > 0.8660254 -0.5 0.0 > > 0.0 0.0 8.0 > > %endblock LatticeVectors > > > > AtomicCoordinatesFormat NotScaledCartesianAng > > > > %block AtomicCoordinatesAndAtomicSpecies > > -0.000007 0.000017 -0.127708 1 12.01 > > 1.419987 0.000012 -0.127709 1 12.01 > > -0.000021 -0.000010 3.725312 1 12.01 > > 1.419990 -0.000005 3.725319 1 12.01 > > %endblock AtomicCoordinatesAndAtomicSpecies > > > > SuperCell_1 3 > > SuperCell_2 3 > > SuperCell_3 0 > > > > AtomicDispl 0.2 Bohr > > > > BandLinesScale pi/a > > %block BandLines > > 1 0.000 0.000 0.000 \Gamma > > 173 1.1547005 0.000 0.000 M > > 100 1.1547005 0.666666667 0.000 K > > 200 0.000 0.000 0.000 \Gamma > > %endblock BandLines > >
