vasp 5.2.2 Elastic Constants
Posted: Mon Jul 20, 2009 8:52 am
I am looking at the output for the elastic constants but I'm getting some inconsistencies.
I have set ISIF=3 and IBRION=6 in my INCAR file in order to calculate elastic constants of a pre-relaxed structure (relaxed cell parameters and ionic coords).
I noticed that by default, just two strains for each unit-cell coordinate are applied on either side of the equilibrium. I tried to change that by setting NFREE=4 and now there are four strains on either side of equilibrium but POTIM should be halved in order to have the max/min strains the same as before.
The problem is that the results do not agree with NFREE=4 and the default (no NFREE specified).
Here's an idea of the disagreement (rigid ions, i.e. SYMMETRIZED ELASTIC MODULI) for a slightly tetragonal cell:
NFREE =4:
unrelaxed ions: C_11=C_33=2.18, C_22=2.24, C_12=1.02, C_13=0.68,
C_44=C_55=1.42, C_66=1.09
no NFREE specified
C_11=C_33=2.70, C_22=2.25, C_12=1.36,C_13=1.14,
C_44=C_55=1.25, C_66=0.95
The results for relaxed ions are also in severe disagreement.
The case of no NFREE specified agrees with my independent calculations for this system, obtained by a script which applies strains. Why does NFREE=4 give such strange results?
Many thanks
I have set ISIF=3 and IBRION=6 in my INCAR file in order to calculate elastic constants of a pre-relaxed structure (relaxed cell parameters and ionic coords).
I noticed that by default, just two strains for each unit-cell coordinate are applied on either side of the equilibrium. I tried to change that by setting NFREE=4 and now there are four strains on either side of equilibrium but POTIM should be halved in order to have the max/min strains the same as before.
The problem is that the results do not agree with NFREE=4 and the default (no NFREE specified).
Here's an idea of the disagreement (rigid ions, i.e. SYMMETRIZED ELASTIC MODULI) for a slightly tetragonal cell:
NFREE =4:
unrelaxed ions: C_11=C_33=2.18, C_22=2.24, C_12=1.02, C_13=0.68,
C_44=C_55=1.42, C_66=1.09
no NFREE specified
C_11=C_33=2.70, C_22=2.25, C_12=1.36,C_13=1.14,
C_44=C_55=1.25, C_66=0.95
The results for relaxed ions are also in severe disagreement.
The case of no NFREE specified agrees with my independent calculations for this system, obtained by a script which applies strains. Why does NFREE=4 give such strange results?
Many thanks