I think the reason of the difference is the presence of macroscopic
electric fields that makes the phonon dispersion close to gamma non
-analytic (it depends on the direction of approach to Gamma point).
Accounting for this non-anayticity is the reason why one computes the
effective charges and the dielectric matrix.
when you select the path (with matdyn.x) the code knows which direction
you are approaching Gamma and includes the non analytic terms
accordingly. you get therefore the LO-TO splitting of modes that would
be otherwise degenerate (174.65). The TO mode stays there while the LO
is shifted up to 322). The other modes are not changed because they
don't have a macroscopic polarization.
When calculation the Dos the code does not know which direction to adopt
and does not include the LO-TO splitting .. the TO matrix is
diagonalized that have additional symmetry. This is in principle wrong
but the gamma point is just a single point in the DoS. neighboring
points and the rest of the points in the BZ should be computed correctly.
HTH
stefano
On 15/08/2018 23:24, Jie Peng wrote:
Dear QE users:
I have been running phonon calculations of HfS2 using quantum
espresso. I followed the steps of relaxing to the equilibrium atomic
configuration using pw.x, computing dynamical matrices on a k point
mesh grid using ph.x, producing interatomic force constant matrix
using q2r.x.
Now I want to plot phonon dispersion along high symmetry direction
Gamma-M-K-Gmma in the HCP (hexagonal closed pack) HfS2 lattice. So I
used the following input file for matdyn.x:
/ &input/
/ asr='crystal'/
/ amass(1)=178.49,/
/ amass(2)=32.065,/
/ ntyp=2/
/ flfrc='HfS2.fc', flfrq='HfS2.freq', q_in_band_form=.true.,/
/ //
/ 4/
/ 0 0 0 40/
/0.5 -0.2887 0 40/
/0.6667 0 0 40/
/0 0 0 1/
which gives me phonon frequencies at Gamma point as:
/ diagonalizing the dynamical matrix .../
/
/
/ q = 0.0000 0.0000 0.0000/
/ **************************************************************************/
/ freq ( 1) = -0.000000 [THz] = -0.000001 [cm-1]/
/ ( 0.545987 -0.000000 -0.167781 -0.000000 -0.084152 -0.000000 )/
/ ( 0.545987 -0.000000 -0.167781 -0.000000 -0.084152 -0.000000 )/
/ ( 0.545987 -0.000000 -0.167781 -0.000000 -0.084152 0.000000 )/
/ freq ( 2) = 0.000000 [THz] = 0.000004 [cm-1]/
/ ( 0.166188 0.000000 0.552429 0.000000 -0.023176 -0.000000 )/
/ ( 0.166188 0.000000 0.552429 0.000000 -0.023176 -0.000000 )/
/ ( 0.166188 0.000000 0.552429 0.000000 -0.023176 0.000000 )/
/ freq ( 3) = 0.000000 [THz] = 0.000008 [cm-1]/
/ ( 0.087255 0.000000 -0.002306 0.000000 0.570714 0.000000 )/
/ ( 0.087255 0.000000 -0.002306 0.000000 0.570714 0.000000 )/
/ ( 0.087255 0.000000 -0.002306 0.000000 0.570714 0.000000 )/
/ freq ( 4) = 5.235913 [THz] = 174.651242 [cm-1]/
/ ( 0.123126 0.000000 0.213241 0.000000 -0.000000 -0.000000 )/
/ ( -0.342690 -0.000000 -0.593505 -0.000000 0.000000 0.000000 )/
/ ( -0.342690 -0.000000 -0.593505 -0.000000 0.000000 0.000000 )/
/ freq ( 5) = 7.799484 [THz] = 260.162777 [cm-1]/
/ ( -0.000000 0.000000 0.000000 -0.000000 0.000000 0.000000 )/
/ ( 0.707104 -0.000013 0.001900 -0.000000 -0.000000 -0.000000 )/
/ ( -0.707104 0.000013 -0.001900 0.000000 -0.000000 0.000000 )/
/ freq ( 6) = 7.799484 [THz] = 260.162777 [cm-1]/
/ ( -0.000000 -0.000000 -0.000000 -0.000000 -0.000000 0.000000 )/
/ ( 0.001900 0.000000 -0.707104 -0.000000 0.000000 -0.000000 )/
/ ( -0.001900 -0.000000 0.707104 0.000000 0.000000 0.000000 )/
/ freq ( 7) = 9.162102 [THz] = 305.614822 [cm-1]/
/ ( -0.000000 0.000000 0.000000 0.000000 -0.246235 0.000000 )/
/ ( 0.000000 -0.000000 -0.000000 0.000000 0.685335 -0.000000 )/
/ ( 0.000000 0.000000 -0.000000 -0.000000 0.685335 0.000000 )/
/ freq ( 8) = 9.170687 [THz] = 305.901189 [cm-1]/
/ ( -0.213241 0.000000 0.123126 -0.000000 0.000000 -0.000000 )/
/ ( 0.593505 -0.000000 -0.342690 0.000000 -0.000000 0.000000 )/
/ ( 0.593505 -0.000000 -0.342690 0.000000 -0.000000 0.000000 )/
/ freq ( 9) = 10.162710 [THz] = 338.991522 [cm-1]/
/ ( 0.000000 0.000000 -0.000000 -0.000000 -0.000000 -0.000000 )/
/ ( -0.000000 -0.000000 0.000000 0.000000 -0.707107 0.000000 )/
/ ( -0.000000 -0.000000 -0.000000 0.000000 0.707107 0.000000 )/
/ **************************************************************************/
However, when I tried to compute phonon DOS in which a mesh grid
rather than a list of high symmetry kpoints was declared, a different
set of Gamma point phonon frequencies appeared. The input file for DOS
calculation is:
/ &input/
/ asr='crystal'/
/ amass(1)=178.49,/
/ amass(2)=32.065,/
/ ntyp=2/
/ flfrc='HfS2.fc', flfrq='HfS2_DOS.freq'/
/ dos=.true./
/ fldos='HfS2.dos'/
/ deltaE=3/
/ nk1=30, nk2=30, nk3=30,/
/ //
The Gamma point frequencies are:
/diagonalizing the dynamical matrix .../
/
/
/ q = 0.0000 0.0000 0.0000/
/ **************************************************************************/
/ freq ( 1) = -0.000000 [THz] = -0.000004 [cm-1]/
/ ( 0.025759 -0.000000 -0.007450 -0.000000 -0.576727 -0.000000 )/
/ ( 0.025759 -0.000000 -0.007450 -0.000000 -0.576727 -0.000000 )/
/ ( 0.025759 -0.000000 -0.007450 -0.000000 -0.576727 0.000000 )/
/ freq ( 2) = -0.000000 [THz] = -0.000003 [cm-1]/
/ ( -0.568506 0.000000 -0.097733 0.000000 -0.024130 -0.000000 )/
/ ( -0.568506 0.000000 -0.097733 0.000000 -0.024130 -0.000000 )/
/ ( -0.568506 0.000000 -0.097733 0.000000 -0.024130 0.000000 )/
/ freq ( 3) = 0.000000 [THz] = 0.000005 [cm-1]/
/ ( -0.097317 -0.000000 0.568969 -0.000000 -0.011697 -0.000000 )/
/ ( -0.097317 -0.000000 0.568969 -0.000000 -0.011697 -0.000000 )/
/ ( -0.097317 -0.000000 0.568969 -0.000000 -0.011697 0.000000 )/
/ freq ( 4) = 5.235913 [THz] = 174.651242 [cm-1]/
/ ( -0.000025 0.000000 0.246235 0.000000 -0.000000 -0.000000 )/
/ ( 0.000070 -0.000000 -0.685335 -0.000000 0.000000 0.000000 )/
/ ( 0.000070 -0.000000 -0.685335 -0.000000 0.000000 0.000000 )/
/ freq ( 5) = 5.235913 [THz] = 174.651242 [cm-1]/
/ ( 0.246234 0.000623 0.000025 0.000000 0.000000 -0.000000 )/
/ ( -0.685333 -0.001733 -0.000070 -0.000000 -0.000000 -0.000000 )/
/ ( -0.685333 -0.001733 -0.000070 -0.000000 0.000000 0.000000 )/
/ freq ( 6) = 7.799484 [THz] = 260.162777 [cm-1]/
/ ( 0.000000 -0.000000 0.000000 0.000000 0.000000 0.000000 )/
/ ( 0.707105 -0.000010 0.001673 -0.000000 -0.000000 -0.000000 )/
/ ( -0.707105 0.000010 -0.001673 0.000000 -0.000000 0.000000 )/
/ freq ( 7) = 7.799484 [THz] = 260.162777 [cm-1]/
/ ( 0.000000 -0.000000 -0.000000 -0.000000 -0.000000 0.000000 )/
/ ( 0.001673 0.000000 -0.707105 -0.000000 0.000000 -0.000000 )/
/ ( -0.001673 -0.000000 0.707105 0.000000 0.000000 0.000000 )/
/ freq ( 8) = 9.672630 [THz] = 322.644206 [cm-1]/
/ ( 0.000000 0.000000 -0.000000 0.000000 -0.246235 0.000000 )/
/ ( 0.000000 -0.000000 0.000000 0.000000 0.685335 -0.000000 )/
/ ( -0.000000 0.000000 0.000000 -0.000000 0.685335 0.000000 )/
/ freq ( 9) = 10.162710 [THz] = 338.991522 [cm-1]/
/ ( -0.000000 -0.000000 -0.000000 0.000000 -0.000000 -0.000000 )/
/ ( -0.000000 -0.000000 0.000000 0.000000 -0.707107 0.000000 )/
/ ( 0.000000 -0.000000 -0.000000 0.000000 0.707107 0.000000 )/
/ **************************************************************************/
I summarize the phonon frequencies for easier comparison:
*Gamma-M-K-Gamma:* -0.0000 0.0000 0.0000 174.6512 260.1628
260.1628 305.6148 305.9012 338.9915
*DOS: *-0.0000 -0.0000 0.0000 174.6512 174.6512 260.1628
260.1628 322.6442 338.9915
As can be seen, the phonon calculation along high symmetry point shows
a degenerate frequency at ***260.1628* while the DOS calculation shows
degeneracy at *174.6512 *and *260.1628.* Moreover, a new phonon mode
with frequency *322.6442 *appears in the DOS calculation that is
absent from calculation along high symmetry directions.
Why should the Gamma point frequency depend on the list of kpoints one
is diagonalizing dynamical matrices on? It does not seem to be
numerical error since the difference is so huge. Can anyone help me
understand it? Thank you very much
FYI, input file for ph.x is shown below:
/
phonons of HfS2
&inputph
prefix='Hf'
epsil=.true.
fildyn='HfS2.dyn',
outdir='./tmp/'
tr2_ph=1.0d-12
ldisp=.true
nq1=4
nq2=4
nq3=4
/
/
--
------------------------------------------------------------------------------------------------------------------------
Jie Peng
PhD student
2134 Glenn Martin Hall, Mechanical Engineering, University of Maryland
College Park, Maryland, USA
Phone:(+1) 240-495-9445
Email: [email protected] <mailto:[email protected]>
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