CASTEP notes

Geometry Optimization

SCF 電子結構收斂 ► 若在這無法收斂 ► Oband DFT 要去動SCF相關的參數

步驟

  1. SCF over (每一移動一個原子,就算一次SCF)
  2. 算Force ►作用對象是原子
  3. SCF重做一次
  4. 算Stress應力為0 ► 作用對象為cell
  5. BFGS若不收斂,則為GO state不收斂(但有SCF有收斂)
    ► 最常原因是因為E_cut不夠,尤其在動晶包 (請看Stress是否有小不下來的現象)

長晶包 ► 應力波

force 微分一次(energy對位置微分),原子看force
stress 微分兩次,晶包看stress

臨界阻泥震盪 ► 最快停下來

(2018/02/06)

force 與總個(晶包,內容物)的受力相關(各方向等等)
stress (應變=>伸長量) 長度變化量/參考長度(總長) => “沒有單位”

GO不收斂情況

有幾種,一種是SCF計算,一種是BFGS計算達到次數(預設100次)
要去查看幾個部分。
另外,在判定GO不收斂時,請看Stress值是否持續大而未降下來,以及SCF計算(tolerance)是否有持續漸小。

調控tolerance地方是在:

CASTEP_notes1
CASTEP_notes2
一般medium就夠用,不過還是得考量計算的物性的需求來做決定。

精確度的影響因素

  1. tolerance ► Q:如何判斷他的值該設多少? A:還是要看計算什麼物性來做決定。
  2. stress
  3. force

TPSD (2 points steepest descent)

另外,計算的條件是有固定一軸而另一軸可動時,可使用TPSD代替BFGS的計算,會比較有效率。
TPSD ► 2 points 兩點之意(始=>末),若要做TPSD castep計算,則需把BFGS的字手動更改為TPSD。或者直接於GO more內做更改。

另外,還有一種也可以嘗試,使用LBFGS也不錯。

增加E_cut的時機

不是所有的GO不收斂都適用,增加E_cut時,可以提升波函數的品質,進而可以更好的算出stress值,讓stress能夠小下來。
而如果E_cut不夠,則stress小不下來,就不容易收斂。
所以建議要看stress有沒有持續小不下來的跡象,方建議使用此法解決。

Fractional coordinates

原子的位置表示,是分數座標。
Real Lattice 是(直或橫)的為A、B、C向量,而是由 uA(向量)+vB(向量)+wC(向量)所組成。

舉例:

1
2
3
4
5
6
7
8
9
10
11
12
13
!======================================!
! Real Lattice(A) !
! 2.5448282 -1.4697230 0.0001380 !
! -0.0004034 2.9387474 -0.0001380 !
! 0.0002992 -0.0005183 8.0000000 !
!======================================!
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
x Element Atom Fractional coordinates of atoms x
x Number u v w x
x---------------------------------------------------------------x
x C 1 0.666572 0.333428 0.449079 x
x C 2 0.333428 0.666572 0.550921 x
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

如果今天要計算兩個碳原子的C軸方向高度差計算,則是
0.449079*(0,0,8)
0.550921*(0,0,8)
兩相減則為 0.814736 angstrom。 (這時才有單位)


Other notes

DFT-D means Van-der_Waals.
lo-to ► 用於離子晶體計算,在左右移動上補計算
凡德瓦利是屬於激發態,瞬間對偶極,總的來說就是吸引力
DFT ► 屬基態
dft-dispesion
constraint ► 適用於固定原子 (需打開勾選最下面)
restraint ► 適用於表面重構
BFGS delocalize internals自由度較小


極化率計算

極化率分兩種
1.電場(or電子)
2.離子

算極化率

計算極化率的時候,可先儲存file,再到Efield裡面task,刪去 phonon,留下 efield,再做計算較快

以及做極化率計算時候,把原子fix住之後,就不會算低頻電場的值
(metal不可點選 因為計算電場若使用金屬 會有電流產生)

電場做出來的極化率 沒分頻率 相當於極高頻


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1
charge : 數目

增加一個一電子 -1
減少一個一電子 1