[Wien] optimization of an orthorhombic cell

[Tomas Kana]

Dear Bhamu,
There is an alternative way. You may use the 
OrthoOpt package available at 
http://susi.theochem.tuwien.ac.at/reg_user/unsupported/OrthoOpt/
It proceeds in a similar way Stefaan has suggested in previous 
mail copied below. It first fixes the volume and finds 
a minimum energy depending on two variables: c/a and b/a. 
Then, it moves to the next volume and so on until it reaches
minimum volume. 
HTH 
Tomas 

No, not really. You have three (unit cell) degrees of freedom: E(a,b,c). Or similarly E(V, b/a, c/a)   (with V=a*b*c). The different options in optimize_lapw give you a series of unit cells with either different values of V (and b/a and c/a constant), different values of b/a (and V and c/a constant) and different values of c/a (and V and b/a constant). These are three slices through the energy landscape. By finding the energy minimum along one slice, fix it, then go on with the next slice, etc. … you can gradually approach the absolute energy minimum. It might take several iterations through this scheme if your starting cell is far away from the minimum. Best is to start first with the degree of freedom that affects energy most (usually the volume).

 

Stefaan

Předmět: Re: [Wien] optimization of an orthorhombic cell
Dear List,
Could you please help me to get optimize lattice 
parameters from the procedure that I followed as 
mentioned below:
The procedure is a four-step task what I am following:
The original lattice parameters were: a, b, c=16.763005, 24.803600, 16.320431     vol 6785.64777697697 bohr^3.
1.   VARY C/A RATIO with CONSTANT VOLUME and B/A (orthorh lattice)
getting below optimized data: 
 ==========================================================
Lowest point on X-axis is =  -1.615800
Minimum value of B/A is =     1.455755
Minimum value of ENE is =   -339059.104498 Ry
 ==========================================================
Value of A is =     16.85428 bohr  ;    8.91890 Ang
Value of B is =     24.53569 bohr  ;   12.98373 Ang
Value of C is =     16.40929 bohr  ;    8.68342 Ang
 ==========================================================
Initial value of  B/A is =          1.479663
Conv. Unit Cell Vol =  6785.7557 bohr^3 ; **********Ang^
2.  Volume optimization using option 1 in "x optimize" and then calculating new volume from eplot option.
Query: Step two is giving optimized volume but I do not know how to calculate lattices parameters from a single volume
value for the next step "VARY C/A RATIO with CONSTANT VOLUME and B/A (orthorh lattice)"?
The original cell volume and lattice parameters are mentioned at the beginning.
and the new volume from step two is: 6815.3427 bohr^3
The above same query is for the remaining process:
3. optimization of C/A 
4. optimization of V.
*5 At the final step I will do structural 
relaxation (min_.....).
Could you please advice me how to proceed?
Kind regards,
Bhamu