The ABINIT Users Mailing List ( CLOSED )

[Riad Shaltaf <shaltaf@photon.physics.metu.edu.tr>]

Hi..

I think the problem is that your cell expanded very much after the first
optemization step which makes the
maximum nomber of plane waves included in the calculation through the
value of dilatmx is not enuogh to cover the expanded cell. I think if you
start from larger lattice parameter or if you give larger value for
dilatmx, you may solve this problem.  

I hope this might help.
On Sun, 30 Mar 2003, M.T.Storr wrote:

> Hi,
> 
> thanks to those people who answered my questions on the number of bands in 
> a calculation. The question has now been resolved and replaced by several 
> further questions!
> I have run through the study of ecut, then the number of kpoints for my 
> system and so am now at the point of running a geometry optimisation, 
> however I am running into problems with ecut/boxcut.
> I have chosen an ecut of 50 Ha and a kpoint grid as follows:
> 
> #Definition of the k-point grid
> kptopt 1          # Option for the automatic generation of k points, 
> taking
>                   # into account the symmetry
> ngkpt 6 6 6       # Definition of the different grids
> 
> nshiftk 4         # repeated four times, with different shifts
> 
> shiftk 0.5 0.5 0.5
>        0.5 0.0 0.0
>        0.0 0.5 0.0
>        0.0 0.0 0.5
>                   # In cartesian coordinates, this grid is simple cubic, and
>                   # actually corresponds to the
>                   # so-called 4x4x4 Monkhorst-Pack grid
> 
> However when I try to run the geometry optimisisation using:
> 
> optcell 2    # Optimise Nuclear Positions only.
> ionmov 3     # Broyden-Fletcher-Goldfarb-Shanno minimization (BFGS).
> ntime 10     # Number of BFGS steps.
> dilatmx 1.15
> ecutsm 0.0
> 
> Then when the program goes through the input data and echoes the dataset 
> it uses and ecut of:
> 
>  getcut: wavevector=  0.0000  0.0000  0.0000  ngfft=  40  40  40
>          ecut(hartree)=     66.125   => boxcut(ratio)=   2.14285
> 
> whihc is not the 5o Ha that I requested.
> However the program goes ahead and runs the Broyden Step 0, however at the 
> end of this step the program then goes to Broyden Step 1 and then selects 
> the Ecut as requested of 50 Ha:
> 
>  getcut: wavevector=  0.0000  0.0000  0.0000  ngfft=  40  40  40
>          ecut(hartree)=     50.000   => boxcut(ratio)=   0.67925
> 
> However as can be seen the boxcut is now below 1 and I get the following 
> error:
> 
>  getcut: ERROR -
>   Choice of acell, ngfft, and ecut
>   ===> basis sphere extends BEYOND fft box!
>   Recall that boxcut=Gcut(box)/Gcut(sphere)  must be > 1.
>   Action : try larger ngfft or smaller ecut.
>   Note that ecut=effcut/boxcut**2 and effcut=   23.068977
> 
> The question is what am I doing wrong. I would like to use an ecut of 50 
> Ha, and have therefore tried increasing ngfft however I tried 50 50 50 
> and got a boxcut(ratio)=0.84940. This suggests that I would need to 
> increase ngfft to a rather large value just to get the calculation to run.
> Therefore I would like to know why for the Broyden step 0 the program 
> appears to select its own value of ecut which when used to intialise 
> variables such as ngfft etc means that at step 1 when it uses the correct 
> value all the initialised variables are wrong. Any suggestions how to 
> solve this would be great.
> 
> Many Thanks
> 
> Mark
> 
> -- 
> Mark T. Storr
> Research Fellow
> School of Chemistry
> University of Reading
> PO Box 224
> Whiteknights
> Reading
> Berkshire
> RG6 6AD
> Tel: +44 (0)118 9875123 Ext. 7415
> E-mail: M.T.Storr@reading.ac.uk
> 
>