My Community

Dear VASP users,

I am new to VASP, and during my attempts to do relaxation calculations, I have found one problem
as below:

Since fcc structure is geometrically equivalent to a bct (body centered tetragonal), I thought by using the bct structure, a lot of computational time can be saved. However, if I try to relax the bct
structure derived from the relaxed fcc structure, the relaxed c/a ratio for the bct structure will
deviate from the ideal retio (sqrt(2)). Meanwhile, the relaxed lattice constants for the bct structure
are different from that of the fcc structure.

Taking fcc Al for an example,

1) for the fcc structure, on a 6X6X6 kmesh, I got relaxed cubic lattice constant a= 4.0490482967412706 Angstrom.

2) for the bct structure, on a 9X9X6 kmesh, I got relaxed lattice constants
a= 2.8754626531991820 Angstrom, c= 4.0082250390586651 Angstrom

3) for the bct structure, increasing kmesh does not help much. For instance, on a
17X17X12 kmesh, the relaxed lattice constants are
a= 2.8397076051927210 Angstrom, c= 4.0842754510901251

Would you please be kind to tell me what the problem is and how could I get it fixed?
I really appreciate any suggestions from you.

Regards,

Hongbin

------------------------------------------------------------------------------------
Here is the INCAR I used:

general:
System = fccAl

#------ machine ---

LPLANE = .TRUE.
LSCALU = .FALSE.
NSIM = 4
NCORE = 16

EDIFF = 0.000001
EDIFFG = -0.0001
ENCUT = 550

NELM = 100

NSW = 150
IBRION = 1
ISIF = 3
#POTIM = 0.1

ISTART = 0

#ISMEAR= -5
ISMEAR= 1
SIGMA=0.04

PREC = Accurate

AMIX = 0.1
BMIX = 0.00001
AMIX_MAG = 0.2
BMIZ_MAG = 0.00001
#--------------------------------------------------

ALGO = NORMAL #NORMAL FAST VERY_FAST
LORBIT = 11


LMAXMIX = 6

LREAL = AUTO ! evaluate projection operators in real space

-----------------------------------------------------------------------------
And here is the POSCAR file for the bct structure:

bctAl
1.00000000000000
2.8397076051927210 0.0000000000000000 0.0000000000000000
0.0000000000000000 2.8397076051927210 0.0000000000000000
0.0000000000000001 0.0000000000000000 4.0842754510901251
Al
2
Direct
0.0000000000000000 0.0000000000000000 0.0000000000000000
0.5000000000000000 0.5000000000000000 0.5000000000000000

0.00000000E+00 0.00000000E+00 0.00000000E+00
0.00000000E+00 0.00000000E+00 0.00000000E+00

-----------------------------------------------------------------------------
And here the POSCAR for the fcc structure:

fccAl
1.00000000000000
4.0490482967412706 0.0000000000000000 0.0000000000000000
0.0000000000000000 4.0490482967412706 0.0000000000000000
0.0000000000000000 0.0000000000000000 4.0490482967412706
Al
4
Direct
0.0000000000000000 0.0000000000000000 0.0000000000000000
0.0000000000000000 0.5000000000000000 0.5000000000000000
0.5000000000000000 0.0000000000000000 0.5000000000000000
0.5000000000000000 0.5000000000000000 0.0000000000000000

0.00000000E+00 0.00000000E+00 0.00000000E+00
0.00000000E+00 0.00000000E+00 0.00000000E+00
0.00000000E+00 0.00000000E+00 0.00000000E+00
0.00000000E+00 0.00000000E+00 0.00000000E+00

<span class='smallblacktext'>[ Edited ]</span>

Hi,

We're sorry that we didn’t answer your question. This does not live up to the quality of support that we aim to provide. The team has since expanded. If we can still help with your problem, please ask again in a new post, linking to this one, and we will answer as quickly as possible.

Best wishes,

VASP