# [Wien] GMAX = 15 causes xcpot3 error in LAPW0

Laurence Marks L-marks at northwestern.edu
Tue Oct 9 14:37:55 CEST 2007

```The size of the FFT parameters are set in case.in0 with default values
when you initialize the calculation, in your case to 32 32 32. This is
for the default GMAX=12 (unless you have H's in which case it is
presumably GMAX=20). A simple fix is to edit the last line of case.in0

-1 -1 -1    2.00    min IFFT-parameters, enhancement factor

The "-1" tells lapw0 to work out itself what to use depending upon the
maximum k-value it finds and then use a times 2 enhancement (really an
oversampling in reciprocal space).

Alternatively you can put 32*16/12 i.e. 43 (rounded up) instead of 32.

On 10/9/07, Thomas Claesson <tcl at kth.se> wrote:
> Dear Wien users and developers!
>
> My question concerns the parameter GMAX. I have seen a comment on the
> mailing list about the importance of not using a too small value of GMAX
> in order to avoid discontinuities in the charge density at the sphere
> boundary. With an RKmax value of 7.0 I increased GMAX to from 12 to 15,
> which resulted in the following error in LAPW0:
>
>  'SETFF1' - ifft too small in xcpot3
>  'SETFF1' - 2*(KKK+1) LARGER THAN IFFT PARAMETERS IN XCPOT3
>  'SETFF1' - KKK=   0   0  16
>  'SETFF1' - IIx=   0   0  16
>  'SETFF1' - IFFT=  32  32  32
>
> So I would like to know, why can't I use use GMAX > 14 in my case? I have
> tried to increase RKmax to 8.0 but that had no effect on the problem with
> GMAX. Are the possible  values of RKmax and Gmax related to each other? If
> so, how can I determine which RKmax value to use for a specific choice of
> GMAX?
>
>
> Best regards, Thomas Claesson
>
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>

--
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Commission on Electron Diffraction of IUCR
www.numis.northwestern.edu/IUCR_CED
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