[Pw_forum] Help! phonon calculation at G point.

[hailin yu]

From: yuhailin_79 at msn.comTo: pw_users at pwscf.orgSubject: Help! phonon calculation at G point.Date: Wed, 17 Oct 2007 11:08:51 +0800


Dear all:        I have some trouble with phonon calculation at G point. Recently, I have calculated two kinds of PtC, rosksalt structure and zinc-blende structure. Firstly, I have fully relaxed the lattice constants and atomic positions using quantum-espressso3.2.3. The results we attained is well agreement with these results using by VASP and CASTEP. Following, I calculate the phonon frequency at G point of the two structures. For zinc-blende structure, we attained the phonon frequency as follows:    q = (    0.000000000   0.000000000   0.000000000 ) **************************************************************************     omega( 1) =       1.086480 [THz] =      36.241319 [cm-1] (  0.340996  0.000000  0.272050  0.000000 -0.549845  0.000000 ) (  0.346068  0.000000  0.276096  0.000000 -0.558022  0.000000 )     omega( 2) =       1.086480 [THz] =      36.241319 [cm-1] ( -0.191604  0.000000 -0.550378  0.000000 -0.391141  0.000000 ) ( -0.194454  0.000000 -0.558563  0.000000 -0.396958  0.000000 )     omega( 3) =       1.086480 [THz] =      36.241319 [cm-1] (  0.582777  0.000000 -0.340135  0.000000  0.193129  0.000000 ) (  0.591444  0.000000 -0.345194  0.000000  0.196001  0.000000 )     omega( 4) =      16.225674 [THz] =     541.233828 [cm-1] ( -0.026894  0.000000  0.048005  0.000000  0.029340  0.000000 ) (  0.430439  0.000000 -0.768321  0.000000 -0.469592  0.000000 )     omega( 5) =      16.225674 [THz] =     541.233828 [cm-1] ( -0.036407  0.000000  0.009944  0.000000 -0.049641  0.000000 ) (  0.582692  0.000000 -0.159157  0.000000  0.794513  0.000000 )     omega( 6) =      16.225674 [THz] =     541.233828 [cm-1] (  0.042894  0.000000  0.038539  0.000000 -0.023738  0.000000 ) ( -0.686516  0.000000 -0.616817  0.000000  0.379926  0.000000 ) **************************************************************************I don't know why omega(1), omega(2) and omega(3) are not equal to zero? while the optic modes is well agreement with the results attained by VASP. I want to know how to improve the accuracy. Can you tell me? Thanks a lot! For roskslt structure, we attained the phonon frequency as follows:     q = (    0.000000000   0.000000000   0.000000000 ) **************************************************************************     omega( 1) =     -16.323420 [THz] =    -544.494287 [cm-1] (  0.057067  0.000000  0.023583  0.000000  0.002155  0.000000 ) ( -0.921868  0.000000 -0.380961  0.000000 -0.034805  0.000000 )     omega( 2) =     -16.323420 [THz] =    -544.494287 [cm-1] ( -0.015312  0.000000  0.032457  0.000000  0.050295  0.000000 ) (  0.247345  0.000000 -0.524309  0.000000 -0.812467  0.000000 )     omega( 3) =     -16.323420 [THz] =    -544.494287 [cm-1] (  0.018065  0.000000 -0.046988  0.000000  0.035822  0.000000 ) ( -0.291827  0.000000  0.759046  0.000000 -0.578678  0.000000 )     omega( 4) =       0.284260 [THz] =       9.481955 [cm-1] (  0.369808  0.000000 -0.578558  0.000000  0.160509  0.000000 ) (  0.371847  0.000000 -0.581748  0.000000  0.161394  0.000000 )     omega( 5) =       0.284260 [THz] =       9.481955 [cm-1] (  0.598650  0.000000  0.340881  0.000000 -0.150560  0.000000 ) (  0.601951  0.000000  0.342761  0.000000 -0.151390  0.000000 )     omega( 6) =       0.284260 [THz] =       9.481955 [cm-1] ( -0.045937  0.000000 -0.215223  0.000000 -0.669940  0.000000 ) ( -0.046191  0.000000 -0.216410  0.000000 -0.673634  0.000000 ) **************************************************************************Why the phonon frequency is negative? As a test, I have caculated the elastic constants of this structure and find it satisfy formula:C44>0, C11>|C12|,C11+2C12>0. So we don't know what was wrong. Can you tell me? Thanks!The input file as shown in the follow:RS.scf.in:  &control    calculation='scf'    restart_mode='from_scratch'    tstress = .true.    tprnfor = .true.    prefix='RS'    etot_conv_thr=1.0D-5    forc_conv_thr=1.0D-4    pseudo_dir = '/home/yhl/espresso-3.0/pseudo/'    outdir='/home/yhl/tmp/' / &system    ibrav=  2, celldm(1) =8.98, nat= 2, ntyp= 2    ecutwfc =100.0    ecutrho =500.0    degauss =0.02    smearing='mp'    occupations ='smearing' / &electrons    conv_thr =  1.0d-12    mixing_beta = 0.7 /ATOMIC_SPECIES Pt  195.08 Pt.pbe-nd-rrkjus.UPF C   12.01  C.pbe-rrkjus.UPFATOMIC_POSITIONS Pt 0.00 0.00 0.00 C  0.5 0.5 0.5K_POINTS AUTOMATIC  12 12 12 1 1 1RS.ph.in  phonon of PtC&inputph  tr2_ph=1.0d-14,  prefix='RS',  ldisp=.true.,  nq1=4,nq2=4,nq3=4  amass(1)=195.08,  amass(2)=12.01,  outdir='/home/yhl/tmp/',  fildyn='RS.dyn', /

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