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Hello

i have a question about the "LATTYP" Routine, as included as output in the OUTCAR.
I am doing some research in binary 2D Alloys and i have a POSCAR like this for a monoclinic A4B2-System:
(the Results are without any Relaxation: ISIF=2 IBRION=-1 NSW=0 and with ISMEAR = 1 & SIGMA = 0.1)


##########################
1)
POSCAR:
-Volume
6 0 0
2 1 0
0 0 6.11374298681587706
4 2
C
0 0 0 A
1 0 0 A
2 0 0 A
3 0 0 A
4 0 0 B
5 0 0 B

KPOINTS
Gamma
4 10 1

Result:
F= -.37496896E+02 E0= -.37497636E+02 d E =0.222051E-02 mag= 14.7314
##########################

if i go in the OUTCAR, the LATTYP Routine give me this curious cell:
##########################
2)
-Volume
-4 1 0
0 0 -6.11374298681587706
6 -3 0
4 2
C
0 0 0 A
1 0 0 A
2 0 0 A
3 0 0 A
4 0 0 B
5 0 0 B

KPOINTS
Gamma
8 1 5

if i calculate this i get:
F= -.37600742E+02 E0= -.37600401E+02 d E =-.102088E-02 mag= 14.3858
##########################
A REALLY BIG DIFFERENCE !!!

and NOW the OUTCAR gives me another different unitcell:
##########################
3)
(-4 1 0)
(0 0 6.1137429..)
(6 0 0)
##########################
which i didn't calculate




What i tried was to see what VASP will give me as a Result and as a new LATTYP-cell
so i created cell #4). I shifted the positions of the Atoms by 0.125 in the x & y-direction
because till now i thought maybe VASP doesn't like unitcells where the all of the Atoms
are on the lattice vectors

##########################
4)
POSCAR:
-Volume
2 1 0
-2 2 0
0 0 6.11374298681587706
4 2
C
0.125 0.125 0 A
1.125 0.125 0 A
2.125 0.125 0 A
3.125 0.125 0 A
4.125 0.125 0 B
5.125 0.125 0 B

KPOINTS:
Gamma
7 5 1

Results:
F= -.37522427E+02 E0= -.37521831E+02 d E =-.178692E-02 mag= 14.5918
##########################
again a difference in Energy and magnetization!

here i get in the OUTCAR this totally new Lattice:
##########################
5)
(0 -3 0)
(0 0 -6.11374298681587706)
(2 4 0)
and if I calculate this as
POSCAR:
-Volume
0 -3 0
0 0 -6.11374298681587706
2 4 0
4 2
C
0.5 0 0 A
1.5 0 0 A
2.5 0 0 A
3.5 0 0 A
4.5 0 0 B
5.5 0 0 B

KPOINTS:
Gamma
7 1 5

Result:
F= -.37489317E+02 E0= -.37488782E+02 d E =-.160413E-02 mag= 14.8550
##########################






SO MY QUESTIONS ARE:
1) which Lattice is "the right one" ? or the preffered one to be calculated?
2) IF i put a lattice and positions in the POSCAR and the "LATTYP" Routine
gives another OUTPUT in the OUTCAR, is MY POSCAR-file used or the LATTICE
which LATTYP finds?? And if the LATTICE from LATTYP is used, what happens with the KPOINTS??
3) How can I tell VASP to keep my lattice vector and atom positions?

THANK YOU IN ADVANCE

VASP uses the geometry input given in POSCAR for the calculation, the symmetry information given in OUTCAR just informs about the internal vasp symmetry analysis (which re-arranges the lattice vectors, but is not used further on, except for the space group information of the lattice).
Total energies only may be compared if the
-- lattice types, shapes (including the angles between the lattice vectors!) and volumes match, and the corresponding k-point sets are equivalent
-- if all calculations are converged o the same level of accuracy electronically
-- atoms are kept fixed if IBRION=-1, or if you use the selective dynamics option in POSCAR and keep (some of) the atoms fixed

But what u mean with:
i can compare only the Total Energy if the angles btw lattice vectors are the same?
The POSCARs i included here are all for the SAME Structure and describe the same structure!!!
Why do i get different Energies despite that the Structure are everytime the same and the KPOINT set is equivalent!?

I think the angle between the e.g. the lattice vectors a,b and a,c of the Bravais matrices
2 1 0
-2 2 0
0 0 6.11374298681587706

and

0 -3 0
0 0 -6.11374298681587706
2 4 0

differ slightly