[Wien] optimization of an orthorhombic cell

Fri Oct 13 14:10:55 CEST 2017

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

Van: Wien [mailto:wien-bounces at zeus.theochem.tuwien.ac.at] Namens Dr. K. C. Bhamu
Verzonden: vrijdag 13 oktober 2017 14:04
Aan: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
Onderwerp: Re: [Wien] optimization of an orthorhombic cell

Thank you very much for a hint.
I thought the value obtained from "VARY C/A RATIO with CONSTANT VOLUME and B/A (orthorh lattice)"  are well optimised then why I do need to optimise all of them again.

Now I see, B/A scaling will give us optimised b, then C/A scaling will give us optimised C and finally optimised v(=a*b*c) will give us optimised "a".
Using optimised a,b,c structure relaxation will be done.

I should be fine to optimised an orthorhombic cell.   Is it?

Regards
Bhamu

On Fri, Oct 13, 2017 at 11:18 AM, Fecher, Gerhard <fecher at uni-mainz.de<mailto:fecher at uni-mainz.de>> wrote:
you have three quantities V=a*b*c, b=a*(b/a) and c=a*(c/a)
so what is the problem ?

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at<mailto:wien-bounces at zeus.theochem.tuwien.ac.at>] im Auftrag von Dr. K. C. Bhamu [kcbhamu85 at gmail.com<mailto:kcbhamu85 at gmail.com>]
Gesendet: Freitag, 13. Oktober 2017 00:42
An: A Mailing list for WIEN2k users
Betreff: 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

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