关注：

1） 虚频的起因；

2） 如何分析虚频由哪些原子朝哪些方向的振动引起的；

3） kpoints 或k-mesh密度如何通过影响自洽计算精度来影响声子的计算

4）  反铁磁或铁磁参数设置后，优化计算通常会改变晶格参数，采用优化后改变了的晶格参数计算声子谱？

5） DFPT计算声子谱可以不扩胞，只需要提高k-mesh密度(如0.03)即可，这比扩胞后， 采用粗网格(k-mesh=0.04或0.05)计算效果还要好。

6） 一些参数设置 NDOWN,

1.  INCAR分析：

System=PuH3
PREC=Accurate
GGA=PE
ISMEAR=0
SIGMA=0.05
ENCUT=550
EDIFF=0.1E-5
EDIFFG=-0.01
LREAL=.FALSE.
IBRION=8
IALGO=38
LORBIT=11
LDAU=.TRUE.
LMAXMIX=6
LDAUTYPE=2
LDAUL=  -1  3
LDAUJ= 0.0  0.51
LDAUU= 0.0  4.5
LASPH=.TRUE.
ISPIN=2
MAGMOM= 0 0 0 0 0 0 2 2
KSPACING=.251327412287  【数值很奇怪哈！】

2. 参数解释：

（1） KSPACING  无KPOINTS文件情形

Default:    
KSPACING = 0.5
KGAMMA = .TRUE.

The tag  KSPACING determines the number of k-points if the KPOINTS file is not present (see Sec. 5.5). KSPACING is the smallest allowed spacing between k-points in units of Å. The number of k-points increases when the spacing is decreased. The number of k-points in the direction of the first, second and third reciprocal lattice vector is determined by the equations

（3）LORBIT 计入分波声子密度
LORBIT = .TRUE. | .FALSE. (VASP.3.2)
LORBIT = 0 | 1 | 2 | 5 | 10 | 11 | 12 (VASP.4.X and later)

This flag determines, together with an appropriate RWIGS (see section 6.33), whether the PROCAR  or PROOUT  files (see section 5.21) are written.

The file PROCAR contains the spd- and site projected wavefunction character of each band. The wavefunction character is calculated, either by projecting the orbitals onto spherical harmonics that are non-zero within spheres of a radius RWIGS around each ion (LORBIT=1, 2), or using a quick projection scheme relying that works only for the PAW method (LORBIT=10,11,12, see below).

If the LORBIT flag is not equal zero, the site and l-projected density of states is also calculated.
If the projector augmented wave method is used, LORBIT can also be set to 10, 11 or 12. This alternative setting selects a quick method for the determination of the spd- and site projected wave function character and does not require the specification of a Wigner-Seitz radius in the INCAR file (the RWIGS line is neglected in this case). The method works only for PAW POTCAR files and not for ultrasoft or norm conserving pseudopotentials

（4）NUPDOWN ：强制设置自旋向下电子数为0，则近似认为是AFM设置？
NUPDOWN [integer] (up from VASP.4.X)

Allows calculations for a specific spin multiplet, i.e. the the difference of the number of electrons in the up and down spin component will be kept fixed to the specified value.

There is a word of caution required: If NUPDOWN is set in the INCAR file the initial moment for the charge density should be the same. Otherwise convergence can slow down.

When starting from atomic charge densities (ICHARG=2), VASP will try to do this automatically by setting MAGMOM to NUPDOWN/NIONS. The user can of course overwrite this default by specifying a different MAGMOM  (which should still result in the correct total moment).

If one initializes the charge density from the one-electron wavefunctions, the initial moment is always correct, because VASP ``pushes'' the required number of electrons from the down to the up component. Initiallizing the chargedensity from the CHGCAR file (ICHARG=1), however, the initial moment is usually incorrect!

kpoints 或k-mesh密度测试：

The three qualities of k-point separation for CASTEP (1/Angstrom) >>>

          coarse.le..08 & medium.le..05 & fine.le..04

Please input the quality of Monkhorst-Pack grid (Default is .035) >>>

0.04

（1）the quality of Monkhorst-Pack grid=0.045

K-Points

0

Monkhorst Pack

 3   3   5

 0   0   0

447 f/i=    0.002526 THz     0.015872 2PiTHz    0.084261 cm-1     0.010447 meV

448 f/i=    0.005017 THz     0.031522 2PiTHz    0.167345 cm-1     0.020748 meV

449 f/i=    5.995156 THz    37.668674 2PiTHz  199.976860 cm-1    24.793980 meV

450 f/i=    7.474510 THz    46.963733 2PiTHz  249.322818 cm-1    30.912101 meV

（2）the quality of Monkhorst-Pack grid=0.04

K-Points

0

Monkhorst Pack

 4   4   6

 0   0   0

447 f/i=    0.002744 THz     0.017243 2PiTHz    0.091541 cm-1     0.011350 meV

448 f/i=    0.003664 THz     0.023023 2PiTHz    0.122227 cm-1     0.015154 meV

449 f/i=    4.739132 THz    29.776843 2PiTHz  158.080416 cm-1    19.599481 meV

450 f/i=    7.669601 THz    48.189524 2PiTHz  255.830343 cm-1    31.718931 meV

网络摘录：尊重版权，采用实名制

刘立仁-新师大(275560060) 2013/10/30 4:42:30

前提是你确定给出的是一个晶胞还是两个晶胞
如果给出的是z轴方向的两个晶胞呢
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 4:43:25
那就是0，1，1呗
湘大-杨振华(421065740) 2013/10/30 4:43:47
这是一个晶包
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 4:44:10
不过，纵使它是2倍的超胞，我也是针对这个超胞来算的，这个超胞的0,1,2晶面
刘立仁-新师大(275560060) 2013/10/30 4:45:56
我看成两个了，所以说011
周大伟-南师(16058984) 2013/10/30 6:34:24
phonopy 算过的声子谱线上如何找到原子按着虚频移动的方向，有结果数输出吗，有人做过吗。
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 6:34:48
用一个软件
可以看到
周大伟-南师(16058984) 2013/10/30 6:34:59
哦，VI_SIM？
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 6:35:05
嗯
周大伟-南师(16058984) 2013/10/30 6:35:23
能给出移动后的结构吗，
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 6:35:33
能呀
周大伟-南师(16058984) 2013/10/30 6:35:54
我算一个二维结构，总是虚频。。不知道哪里出错了
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 6:36:49
你想看看哪来的虚频是吧
V_sim可以看的
周大伟-南师(16058984) 2013/10/30 6:37:15
哦，需要在phonopy里面特殊设置那个命令啊
何朝宇湘潭大学<hanyuhechaoyu@163.com> 2013/10/30 6:37:50
不需要呀，后处理的时候用一个命令就生成
周大伟-南师(16058984) 2013/10/30 6:38:11
哦，什么命令？