HCP has two parameters to optimize namely a and a/c; the process detailed here gives pretty accurate result to determine those parameters. Zr is taken as an example here, where positions are special, high symmetry positions, i.e. (1/3, 2/3,1/4) and (2/3, 1/3,3/4)
i. Take 7 to 8 different volumes across the estimated equilibrium volume (from published XRD data).
ii. Range of volume scanned should be typically within +/- 25%
iii. If atom positions are in high symmetry positions then it is better not to disturb them. However, in POSCAR file selected dynamics and motion should be set T, to get force info in OUTCAR
An example of POSCAR -
HCP Zr parama (vary this to get different volume) 1.000000000 0.000000000 0.00000000000 -0.500000000 0.866025404 0.00000000000 0.000000000 0.000000000 1.60000000000 Zr 2 Selective dynamics Direct 0.6666666666666667 0.3333333333333333 0.7500000000000000 F F F 0.3333333333333333 0.6666666666666667 0.2500000000000000 T T T
iv. INCAR setting
ISTART=0 LREAL=.FALSE. PREC=Accurate ADDGRID=.TRUE. ENCUT=550 ISPIN=2 NPAR=6 MAGMOM=2*2.0
IBRION=2 ISIF=4 POTIM=0.15 ISMEAR=1 SIGMA=0.01
NSW=100 NELM=50 EDIFF=1.0E-6
v. Fit 3rd order BM EOS to determine the equilibrium volume and bulk modulus (V0 and B)
vi. Plot c/a vs. V and do a polynomial fit
vii. For the equilibrium volume (V0) calculate the c/a from the above fit; also calculate equilibrium a from V0 and c/a
viii. Use these new cell parameters to determine the energy accurately using ISMEAR = -5 in INCAR
Hello Mr. Chandra, Chandra Bhanu Basak
I have a few questions:
1) Should i optimize k-points before or after the lattice parameters? If after, which k-points should i use for this optimization?
2)Why use 2.0 as MAGMOM for Zr, i have Cd, which are good starting values? Should i optimize magnetic moments as well? Does Cd need ISPIN=2?
3)Why is there one atom with locked positions (F F F), shouldn't be both trying to relax into the cell?
4)Why is ENCUT = 550? And the optimization of the cut-off energy should be done after this calculation?
Thank you very much, i hope to hear from you soon.
Best regards, Alejandro
1. With the recent computation power people take largw no. of k-points without any optimization :)
2. Always take a higher value of MAGMOM, e.g. For Zr it is actually 0
3. You can free all atoms if you need, these are high symmetry Wyckoff position, so result won't change much. In fact, you shift one atom to 0 0 0 position then it makes more sense to fix that atom.
4. Same as 1, take largest value of ENCUT from all POTCARs and multiply it with anything between 1.3 to 1.5.
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