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justinzhouke

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专业: 凝聚态物性I:结构、力学和

[交流] 石墨烯声子谱Gamma点附近虚频，怎么处理已有4人参与

计算石墨烯的声子谱，计算发现Gamma点附近出现虚频（如下)。不知道怎么处理。

  1 0.000000    0.0000 0.0000 0.0000  879.5294 1560.4160 1560.4160
  2 0.022223    -3.7499 41.7646 65.5168  879.2473 1559.8164 1561.6153
  3 0.044447    -6.5222 83.4849  130.6515  878.3968 1557.9969 1565.0765
  4 0.066670    -6.7089  125.1109  195.0346  876.9649 1554.9026 1570.4069
  5 0.088893    6.6043  166.5984  258.3457  874.9310 1550.4615 1577.0073
  6 0.111117    17.6058  207.9030  320.3191  872.2675 1544.6098 1584.1435
  7 0.133340    29.2947  248.9746  380.7432  868.9412 1537.3218 1591.0273

基本设置包括PP，是按照论文里面来的（PHYSICAL REVIEW B 80, 033406 2009）。
我做过ecut和q-mesh的收敛性测试，q-mesh在16的时候就收敛了。Ecut是55Ry收敛。

In the present analysis, we take advantage of the PWSCF
package of the QUANTUM-ESPRESSO distribution13 to model
the characteristic graphene lattice dynamics. Specifically,
an ultrasoft pseudopotential in the local-density
approximation14 is used with a plane-wave expansion up to
55 Ry and 32x32x1 Monkhorst-Pack meshes adopted for
Brillouin-zone sampling with 0.02 Ry Fermi-Dirac smearing
in the electronic occupation. The calculation also employs
the theoretical lattice parameter a=2.46 Å, which is in excellent
agreement with the experimental value at 300 K. For
bilayer graphene, the Bernal stacking is considered with the
interlayer distance set to the theoretical minimum of
3.36 Å.

输入文件
Gra.scf.in:
1  &control
  2       calculation = 'scf',
  3       restart_mode= 'from_scratch',
  4       pseudo_dir  = '/home/xuzp/bin/lib/pwscf_pot',
  5       outdir    = './tmp',
  6       prefix='graphene',
  7       !disk_io    = 'none',
  8       tstress = .true.
  9       tprnfor = .true.
10       wf_collect=.true.
11 /
12 &system
13    ibrav          = 4
14    celldm(1)    = 4.64873
15    celldm(3)    = 8.13
16    nat          = 2
17    ntyp          = 1
18    ecutwfc       = 55
19    ecutrho       = 550
20    occupations    = 'smearing'
21    smearing       = 'fd'
22    degauss       = 0.02
23 /
24 &electrons
25    diagonalization = 'david'
26    mixing_beta    = 0.7
27    conv_thr       = 1.0d-10
28 /
29 ATOMIC_SPECIES
30 C  12.01078 C.pbe-rrkjus.UPF  !C.pz-vbc.UPF
31 ATOMIC_POSITIONS crystal
32 C 0.6666666666666667 0.3333333333333333 0.5000000000000000
33 C 0.3333333333333333 0.6666666666666667 0.5000000000000000
34 K_POINTS automatic
35 32 32 1 0 0 0

Gra.ph.in:
  1 phonon for graphene
  2 &inputph
  3       tr2_ph       =1.0d-18,
  4       prefix       ='graphene',
  5       epsil          =.false.,
  6       amass(1)       =12.01078,
  7       outdir       = './tmp',
  8       fildyn='graphene.dynG',
  9       !elph=.false.,
10       !trans=.true.,
11       ldisp=.true.
12       nq1=6, nq2=6, nq3=1
13 /

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