[QE-users] Difference in phonon frequencies produced by matdyn.x
Jie Peng
jiepeng at umd.edu
Wed Aug 15 23:24:38 CEST 2018
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 &inputphprefix='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: jiepeng at umd.edu
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