建模中的一些小疑惑

Northword2020年12月19日
  • Materials-Studio
大约 3 分钟

一些乱七八糟的为什么

为什么要建立超胞?

  1. 避免周期性镜像间的互相影响
  2. 考虑多分⼦吸附时的多种构象

如果只计算体性质,用原胞(primitivecell)就可以,建立超胞(supercell)的目的是什么?

我们一般建立晶胞,设置 k 点进行计算,那么建立超晶胞的目的何在呢?

(1)掺杂问题

1)建立超胞是为了获得合适的掺杂浓度;

2)清晰地分成置换固溶体或间隙固溶体

(2)表面吸附问题

1)可以模拟不同晶向的吸附密度;

2)要避免吸附的气体分子和镜像分子间有作用;

https://www.zhihu.com/question/282248524open in new window

如何选择真空层厚度?

todo

如何确定底部保留几层?

todo

埃 Angstrom

埃格斯特朗。长度单位。 $1\text{\AA} = 10^{-10} m = 0.1 nm = 10^{-2}pm$ 。

WiKiPedia: https://zh.wikipedia.org/wiki/埃格斯特朗open in new window

In LaTeX, the command for angstrom is {\AA}.

https://www.aps.org/meetings/abstract/latex.cfmopen in new window

由于 KaTex 尚不支持 \AA ,可用 \text{\AA} 进行渲染。

CIF 文件的区别

Materials Project 上提供四种 CIF 文件,他们本质无区别,所有 CIF 都完全相同或有效等效,它们只是同一晶体结构的不同“设置”。因此,使用哪个都不要紧。

CIF file types - differences? - Materials Project - Materials Science Community Discourse (matsci.org)open in new window

The most important thing is that all the CIFs are either exactly or effectively equivalent, they are just different “settings” of the same crystal structure. So shouldn’t matter too much which you use.

These are:

  • Computed, the structure direct from our calculations, always in the “P1” setting (i.e. no symmetry is detected)
  • Conventional standard, this is the same as “computed” except now the symmetry is detected and it’s put into a conventional setting – this will be more familiar to most scientists, since textbooks and papers etc. will usually report a crystal in its conventional setting
  • Primitive cell, this is the same as “computed” except now it’s reported in its primitive setting (i.e. with the minimum number of atoms required to define the periodic crystal). It’s often useful to use these primitive cells for further calculations, since they contain fewer atoms, but they can be more difficult to visualize – for example, a crystal with cubic symmetry might not have a primitive cell whose angles are 90º. Introductions to crystallography can go into more detail on this.
  • Symmetrized is the almost the same as “computed”, except atomic positions have been moved to their symmetrically equivalent positions if they’re almost but not quite there – this is important e.g. if due to numerical noise an atom may be close to but not exactly at (0,0, 0) even if we know in reality it’s likely to be exactly at (0,0, 0) in that particular setting.

切面用 conventional 还是 primitive ?

Ex42 slab 模型 | Learn VASP The Hard Way (bigbrosci.com)open in new window 评论区:

  1. 切 slab 模型不能用 primitive cell 切,而要用 conventional cell 切。有時候兩種切出來會不一樣
  2. 將 primitive cell 轉換成 conventional cell:利用 Material Studio 的 Build- Symmetry-Find Symmetry 功能,對 primitive cell 的模型“Find Symmetry”,然後“Impose Symmetry”,即可。

刘锦程 MS 建模视频:

切面必须用 conventional cell ,

能带计算必须用 primitive cell ,因为其布里渊区定义正确