Energy vs Vol fitting

[Inaoton]

Hello
        I want to plot Energy Vs Volume for a  hexagonal structure.
     Is it possible to obtain the curve through OptimizeGeometry?
   

[Anders Blom]

Not directly. Depending on your purpose, you may prefer to do an ElasticConstants analysis instead, but otherwise you will just have to very a and c, and plot the total energy and function of the two parameters. Very easy to do in Python, fortunately, you can just make two loops, collect the data, and plot using pylab.

[Inaoton]

So i have to vary one parameter and fix the other and plot the energy Vs lattice parameter and do the same for the other parameter . Is it correct sir?

[Anders Blom]

Yes, but you need to vary both - you can't run a series of calculations where you vary "a" for a single value of "c", you need to do it for all values of "c".

[Inaoton]

Sir,
         For that purpose  i think c/a ratio will be fixed ?..

[Jess Wellendorff]

You're looking at a 2-dimensional optimization problem (there are 2 variables: a and c, or a and c/a). So you are looking for a minimum-energy path in 2D. You therefore need to sample the energy on a 2D grid of (a,c) values.

[Inaoton]

 Sir
          I vary a as well as c for a structure and plotted the volume vs  energy for the primitive cell using LDA . I got a volume which is larger than experimental volume . The kpoints and cut off are well converged for the experimental volume. Should i perform those calculations with higher cut off since i am varying volume each time?

[Jess Wellendorff]

I would first use OptimizeGeometry to find the minimum-energy LDA structure using the computational parameters you use now. Then you will know if your (a,c) sampling results in the "true" LDA result (given all other computational parameters).

[Inaoton]

 I have done using Optimize geometry and for LDA i am getting a=5.89 and c=11.70 (expt a= 5.904 and c=12.03) but volume vs energy fitting gives a=6.054 and c=11.83. why?

[Ulrik G. Vej-Hansen]

It is very hard to say why, when you do not share information about how you did it - please share your files if you want someone else to help you. My guess is that the fit is not as accurate as it should be, but I need to see your script and output to be sure.