problems about optimization of ENCUT ......

[xinhuanwen]

hello everyone

please help me

I am calculating a superlattice system consist of graphene nanoribbons.now I want to know its properties of the ground state,especially its magnetism:pm afm or fm.so for each condition,i will calculate the total energy respectively ，the lowest is the ground state .

First of all . i think i must optimize the ENCUT for each condition .but i only optimize the pm . for my system ENMAX=400ev. during the optimization,the total energy doesn't converge,the difference with different energy cutoffs are about 5or6 mev not 1mev.
kpoints(11 5 1)
360 -41.234745
400 -41.205839
440 -41.186058
480 -41.180300
520 -41.178179
560 -41.180316
600 -41.186082
630 -41.191180
660 -41.196265
690 -41.200828
720 -41.204283
kpoints(11 11 1)
360 -41.234788
400 -41.205852
440 -41.186038
480 -41.180291
520 -41.178186
560 -41.180322
600 -41.186084
630 -41.191185
660 -41.196266
kpoints(13 13 1)
360 -41.276285
400 -41.246915
440 -41.227022
480 -41.221409
520 -41.219294
560 -41.221406
600 -41.227183
630 -41.232272
660 -41.237363
690 -41.241937
720 -41.245368


I didn't obtain the proper ENCUT value.

My questions are:1 is it OK if i only optimize the pm ,not the fm and afm?

2 how can i get the proper ENCUT value ,namely the difference with different energy cutoffs keep around 1mev ?

3 Is the difference with different total energy (eg. the total energy of the PM and the FM for the same superlattice ) absolute? for another superlattice ,i must use the same encut value ?because i want to compare the differences of the different supperlattice (eg. i have two superlatteces A and B. for A:E1=difference with fm and afm of A superlattice ; for B: E2=difference with fm and afm of B superlattice; i want to compare E1 and E2 ) .for A and B superlattice , the k-mesh must be identical?
..............................................................................................................................................................................
optencu.sh
..........................
rm WAVECAR
mpd&
for i in 360 400 440 480 520 560 600 630 660 690 720
do
cat>INCAR<<!
ENCUT=$i
ISTART=0
ICHARG=2
ISMEAR=-5
SIGMA=0.2
NELM=100
!
echo "ENCUT=$i ev"
mpiexec -n 8 vasp
E=$(grep "TOTEN" OUTCAR|tail -1|awk '{printf "%12.6f \n",$5}')
echo $i $E >>comment
done
t.sh
.......................................................................................
POSCAR
..............
8(the width between the nanoribbion) 2(the width of the nanoribbion in the superlattice )
2.465
1 0 0
0 6.928 0
0 0 8
4 2
cart
0 0.577 0
0.5 0.866 0
0.5 1.443 0
0 1.732 0
0 0 0
0 2.308 0
.......................................................................................

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