5 VASP里面的对称性操作

点群问题

0.4 怎样实现HSE06

The cell with the lowest enthalpybelow ～90 GPa, Fddd-BaH6, isillustrated in Figure 4a.55It iscomposed of Ba2+ cations and symmetric H3− counteranions arranged in interwoven herringbone-typepatterns, whose intra-/intermolecular H−H distances measure 0.93/1.86 Å at 70 GPa. The structure’s PBE band gap closes at 60 GPa.

Calculations were also carried out on the 80GPa structure using the HSE06 hybrid functional,54 which has been shown to givemore reliable band gaps in solids, wherein the metallicity was not confirmed via a small density of states (DOS) at theFermi energy. 【怎样实现HSE06?】

0.3 零点能与压力有关系吗？  相同结构在不同压力下零点能会怎样变化？

   Zero-point energy, also called quantum vacuum zero-point energy, is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state.

    从固体结构特点导出的等温状态方程。 这种方法是基础是从详细的固体原子和电子结构知识出发，计算自由能，然后利用该自由能计算状态方程。

     在不考虑温度效应时，该能量主要成分是点阵能和零点能。一般固体的零点能比点阵能小很多，只在键比较弱的稀有气体的凝聚态中才例外。离子晶体的点阵能主要由离子间的库仑作用决定。分子晶体则主要由原子或分子的范德瓦耳斯力决定。平衡这些相互作用的排斥势由实验确定。金属的点阵能由自由电子的动能和交换能、电子和离子之间的库仑能，以及离子间的排斥能之和构成。

　　高温状态方程 　以上的处理都没有考虑温度这一因素，当考虑温度效应时，则必须计入激发态对自由能的贡献。这些激发包括原子热振动、电子和自旋的热激发，以及分子转动等等。由于它们产生了热压力，引出了热状态方程。

0.2 S--d transfer问题可关注Cs、Ba、Ca的相关文章

0.1 PV项对高压相的形成并稳定化到底起什么作用？

    Our calculations suggestthat the stabilization of KH₅ over KH3 under pressure is the resultof increasingthe H−H distances between H₃^-molecules in order to minimize the Coulombrepulsion between them, and in decreasing the volume per hydrogen so as tolower the destabilizing PV contribution to the enthalpy.

1. 如何进行精确的能带结构计算

GW方法

hse方法

1.1 如何指定能带图上的哪条能带属于哪个原子的哪个电子？

2. 振动计算时什么情形下需要考虑非谐效应？ 如何考虑非谐效应？

Metallization of ice is not founduntil p = 4.8 TPa in static calculations (and even higherwhen dynamical effects are estimated at the level ofthe harmonic approximation), when a C2∕m structure,this related to a recently suggested Cmcm structure, becomes the most stablephase.

3. 优化如VASP优化的内部过程是怎样的？哪些参量的设置会显著影响优化结果？

  三元相图的计算

  不同k点产生方式对计算能量如焓变影响有多大？

3.1 VASP优化时各收敛参数的含义、单位及其大小对计算时间的影响。

EDIFF = allowed error in total energy

Default : 10−4

Specifies the global break condition for the electronic SC-loop. The relaxation of the electronic degrees of freedom will be  stopped if the total (free) energy change and the band structure energy change (’change of eigenvalues’) between two steps are both smaller than EDIFF.

   For EDIFF=0, NELM electronic SC-steps will always be performed.

Mind: In most cases the convergence speed is exponential. So if you want the total energy significant to 4 figures set EDIFF to 10−4. There is no real reason to use a much smaller number.（一般情况下没有必要设置更小的值）

单位：

EDIFFG = break condition for the ionic relaxation loop

Default : EDIFF*10

   EDIFFG defines the break condition for the ionic relaxation loop. If the change in the total (free) energy is smaller than EDIFFG between two ionic steps relaxation will be stopped.  

   If EDIFFG is negative it has a different meaning: In this case the relaxation will stop if all forces are smaller than | EDIFFG |. This is usually a more convenient setting.

   EDIFFG might be 0; in this case the ionic relaxation is stopped after NSW steps. EDIFFG does not apply for MD simulations.

摘录：

EDIFF=1E-5是电子自洽步（SCF）的收敛标准，表示0.01meV/cell；
EDIFFG表示离子弛豫的收敛标准，若为正值，表示以能量为收敛标准，意义同上，默认值为EDIFF的10倍；若为负值，表示以离子受力作为收敛标准，单位eV/A，这个标准不能太高，否则很难收敛。
    一般情况下，你可以先设置EDIFFG为正值，跑完后用p4vasp查看一下，能看到能量和力随弛豫步的变化（跟MS类似），能量最低的时候力也基本上就是最小的了，而且对稍微复杂一点的体系，力很难收敛到0.001eV/A。

3.2 log或OSZICAR文件中这几个参数的含义

      N       E                     dE             d eps       ncg     rms          rms(c)

DAV:   1    -0.551005737067E+02   -0.15541E-02   -0.10982E-03 57864   0.552E-01    0.963E-03

DAV:   2    -0.551005848387E+02   -0.11132E-04   -0.13090E-04 58424   0.750E-02    0.551E-03

DAV:   3    -0.551005845920E+02    0.24665E-06   -0.27455E-06 71552   0.136E-02    0.308E-03

DAV:   4    -0.551005843870E+02    0.20505E-06   -0.26526E-07 47128   0.493E-03    0.440E-04

DAV:   5    -0.551005843721E+02    0.14905E-07   -0.51100E-08 31080   0.315E-03

 35 F= -.26270408E+02 E0= -.26270248E+02  d E =-.917801E-05

trial-energy change:   -0.000009  1 .order   -0.000010   -0.000016   -0.000003

step:  13.2930(harm= 13.2930)  dis= 0.00099  next Energy=   -26.270409 (dE=-0.985E-05)

bond charge predicted

摘自handsonI：

initial charge corresponds to the charge of isolated overlapping atoms (POTCAR) for 4 steps the charge remains fixed, then the charge is updated (rms(c) column)

N iteration count

E total energy

dE change of total energy

d eps change of the eigenvalues (fixed potential)

ncg number of optimisation steps H $\psi$

rms total residual vector

rms(c) charge density residual vector

摘自manual:

   Information about convergence speed and about the current step is written to stdout and to the file OSZICAR. Always keep a copy of the OSZICAR file, it might give important information.

  Typically you will get something similar to the following lines:

....

  N is the number of electronic steps,

   E the current free energy,

  dE the change in the free energy from the last to the current step

 and d eps the change in the bandstructure energy.

  ncg the number of evaluations of the Hamiltonian acting onto a wavefunction,

  rms the norm of the residuum ( ) of the trial wavefunctions (i.e. their approximate error) and

  rms(c) the difference between input and output charge density.

  The next line gives information about the total energy after obtaining convergence. 

  The first values is the total free energy F (at this point the energy of the reference atom has been subtracted),

  E0 is the energy for (see section 9.4), and

  d E is the change in the total energy between the current and the last step; for a static run dE is the entropy multiplied by sigma.

For a molecular dynamics (IBRION=0 see section 7.19) this line will is a little bit different:

  1 T= 1873.0 E= -.13382154E+04 F= -.13401522E+04 E0= -.13397340E+04     EK=   .19368E+01 SP=  .00E+00 SK=  .00E+00T corresponds to the current temperature, E to the total free energy (including the kinetic energy of the ions and the energy of the Nosé thermostat). F and E0 have been explained. EK is the kinetic energy, SP is the potential energy of the Nosé thermostat and SK the corresponding kinetic energy.

Additional technical parameters and some status reports are also written to stdout

4.K-mesh密度与q-mesh密度的关系。 为什么q-mesh密度总是小于K-mesh密度？