[Wien] find minimum

[Gavin Abo]

Below is a summary of previous posts that should help.

To plot energy vs b/a:

(a) You should be able to rename the scf files and then plot with the 
energy vs c/a option of eplot 
[http://zeus.theochem.tuwien.ac.at/pipermail/wien/2009-January/012053.html].
(b) You could also plot energy vs b/a in another program like Origin 
[http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg08841.html].

To get the optimized c/a (or b/a) value,  you can calculate it from the 
curve fit equation 
[http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg05826.html].

An example calculation is given below.

When you plot E vs. c/a with eplot, you should get something like:

Fit of:  E = a1 + a2*x + a3*x^2 + a4*x^3 + a5*x^4
a1              = -2201.24
a2              = 0.00233422
a3              = 0.000188276
a4              = 1.33462e-05
a5              = 9.76521e-07

 From dE/dx = 0, we get the equation:

a2 + 2*a3*x+3*a4*x^2+4*a5*x^3 = 0

Solving this equation for x, x = -7.7555, which is the optimized c/a 
ratio in %.  I got x with Octave, but there are other software programs 
that you can find on the internet to solve the polynomial equation.

Octave input:

dEdx = [4*9.76521e-07,3*1.33462e-05,2*0.000188276,0.00233422]
roots(dEdx)

Octave output:

   -1.2474 + 8.6889i
   -1.2474 - 8.6889i
   -7.7555 + 0.0000i  <= This polynomial root is chosen as x, because 
the imaginary part is zero and it appears in the energy vs c/a plot.

If you also want the minimum E, just plug the x value back into the 
curve fit equation and solve for E (= -2201.2495).

On 1/22/2014 7:22 AM, MAHDI SALMANI HIRMAND wrote:
> Dear Prof. Blaha,
> Please let me know why there is not a program for finding minimum of 
> c/a or b/a ratio with Wien2k Package when we optimize c/a or b/a.
> eplot program can show curve of Energy Vs. c/a but it doesn't find 
> minimum of c/a.
> Please guide me
> With best
> Mahdi
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