Structural optimization error

[MANISH ANAND]

Hi abinit user,
I am working on structural optimization for PbTiO3. but every time I am getting boxcut error. even though I am reducing ecut or increasing ngkpt is not working for me. Please help. here is the content of input file.
#Definition of the atom types
ntypat 3
znucl 82 22 8

#Definition of the atoms
natom 5
typat 1 2 3 3 3

#Definition of the planewave basis set
#ndtset 12
ecut 40 Hartree
ecutsm 0.5 Hartree
#Definition of the k-point grid
kptopt 1
occopt 1
ngkpt 8 8 8
nband 17
ngfft 40 40 40
#Definition of the SCF proceed
#toldff 1.0d-8
ionmov 2
dilatmx 2.0
optcell 2
strfact 20
diemac 6.0
toldff 5.0e-6
tolmxf 5.0e-5
nstep 20
ntime 10
# coordinate of the complex
xred
0.0000000000000000 0.0000000000000000 0.0000000000000000
0.5000000000000000 0.5000000000000000 0.5000000000000000
0.0000000000000000 0.5000000000000000 0.5000000000000000
0.5000000000000000 0.0000000000000000 0.5000000000000000
0.5000000000000000 0.5000000000000000 0.0000000000000000

#%% [files]
#%% [paral_info]
#%% max_nprocs = 1
#%% [extra_info]
#%% authors = Unknown
#%% keywords =
#%% descrition = DFT calculation for PbTiO3

and here is the error which I am getting


--- !ERROR
message: |

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= 2.956891
src_file: getcut.F90
src_line: 157
...


leave_new : decision taken to exit ...

[Jordan]

Hi,

It seems you did not define the variable acell which is the lattice parameters a b and c
For a perovkite like PbTiO3 you should have acell = 7.3 7.3 7.3 bohr (check litterature for a better value)

Cheers

[MANISH ANAND]

Hi Jordan.
Thanks for reply. I have used acell =3.89239000 3.89239000 3.89239000. I have just forgot to paste it here. so should I use the acell value which you have suggested?
Thanks.

[ebousquet]

Dear Manish Anand,
Regarding the cell parameters, a good guess is to start with the experimental ones.
For your boxcut problem this is related to the too big dilatmx you defined. Please use something like 1.05 or 1.10.
Your calculations will be faster too, please read the doc regarding dilatmx:
http://www.abinit.org/doc/helpfiles/for ... ml#dilatmx
Best,
Eric