[Wien] Rhombohederal Structure-Boron

[Steven Homolya]

On Sat, 13 Sep 2003 01:41 am, Neerja wrote:
> !!!!!!IMPORTANT!!!!!!
> Our Problem is that, we want to find cold pressure and bulk modulus for
> differnet crystal structures..
>
>
> Dear All,
> I did all calculations for boron (rhomohederal structure).
> Step I instgen_lapw, II: init_lapw III: run_lapw
>
> after this, we get vol of unit cell (V) and energy (E) in Rydberg.
>
> we can calculate Pressure by a general formula P = -dE/DV
>
> and after doing calcuation for pressure for two and three sets of volume,
> we are trying to find bulk mudulus manully by using this relation
> K = V*(dP/dV)
>
> In cae of Boron, we are getting a value 12 times from actual value of bulk
> modulus.
> Kindly suggest us, in what step, we are going wrong ????
>
There could be several reasons:
1) calculating d^2E/dV^2 at a V other than the (numerical) equilibrium volume
2) the fit on the E vs V data used to get the second derivative of E wrt V 
does not correctly describe the function E(V).
3) too much noise in your data (because one or more of your RMT's are too 
big?)

Suggestion:
use the volopt and eosfit utilities to find equilibrium volume and bulk 
modulus.

Unfortunately the use eosfit is not well explained in the user guide, 
especially if you're not using w2web. If you are, then you'll probably just 
need to follow steps as per section 3.6 (QuickStart->VolumeOptimisation) of 
userguide.

If not, you'll need to create a file called case.vol (not case.analysis as the 
userguide seems to suggest). This file should contain your results for E vs 
V, i.e.,
V1 E1
V2 E2
...

Then run
x eosfit

Now, the info you wanted will be in the file case.outputeos, and the fitted E 
vs V curve in case.eosfit. I like to check that it all looks sensible:

gnuplot
gnuplot> plot 'case.eosfit' w l, 'case.vol' w p lt 3 ps 2

Good luck,

Steve

-- 
Steven Homolya
School of Physics and Materials Engineering
Monash University VIC 3800
Australia
Tel: INT +61 3 9905 3694
Fax: INT +61 3 9905 3637