[Wien] Problem in structure optimization

[Gavin Abo]

 From the WIEN2k usersguide [1] and previous posts in the mailing list 
[2,3], I glean that xcpot3 is the exchange-correlation potential in the 
interstitial region [4].


IFFT likely refers to the upper limit of the summation [5] of a fast 
Fourier transform (fft) [6,7].  So by increasing the IFFTx,y,z values in 
the x, y, or z direction or the multiplicative enhancement factor, I 
think you can expect that the fft error reduces for the 
exchange-correlation potential but at the cost of a slow calculation due 
to the increase in the number of fft summation terms.


[1] http://susi.theochem.tuwien.ac.at/reg_user/textbooks/usersguide.pdf

[2] 
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg07410.html
[3] 
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg07166.html

[4] http://susi.theochem.tuwien.ac.at/lapw/index.html

[5] http://www.columbia.edu/itc/sipa/math/summation.html

[6] https://en.wikipedia.org/wiki/Fast_Fourier_transform

[7] 
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg05582.html


On 5/15/2019 10:49 AM, AJAY SINGH VERMA wrote:
> Dear Sir,
>
> I am facing some problem in structural optimization. When i perform my 
> calculation with variable volume percentage from -6 to 6 with 
> difference 1. In starting the complete scf cycle run but when 6% 
> volume term comes error massage shown *"ifft so small in xcpot3".* As 
> per the previous mailing  i change ifft parameter from 75 75 75 to 0 0 
> 0 and enhancement factor from 1.0 to 2.0.in in0 file after that 
> complete cycle run and curve is completely plotted.
> I have some questions:-
> What is the effect of this change?
> does it change the value of crystal parameter?
> what it represent?
>
> Thanking you
> Ajay Singh Verma
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