[Pw_forum] Energy keeps going down
pandey.bramha at gmail.com
Wed Oct 31 14:20:10 CET 2012
On Wed, Oct 31, 2012 at 6:14 PM, david furman <sirok4 at gmail.com> wrote:
> Hi all,
> I sucsessfully calculated equation of state for CaF2 by varying celldm(1)
> in a loop.
> then fitting the EV data to murnaghan EOS to get a parabola like plot.
> Now I try to calculate EV data for CaTiO3 (Pnma space group) by the same
> But, no matter how large the celldm(1) is (keeping the b/a and c/a fixed),
> the energy still goes down instead of going up, with no parabola like
> I assume this is wrong, because when distances between atoms are large
> (expansion of unit cell), this state should be higher
> in energy than the equilibrium volume state, just like when compressing
> the unitcell the energy goes up because of repulsion.
> Can anybody tell me why is this happening ? Could it be a bug ?
Before reaching any excellent conclusion can you provide your input file
> The pseudopotentials i'm using are all PBE
whether it is NCPP or USPP?
> and verified with other systems so they should be OK.
> There are 20 atoms in the primitive unit cell. I'm using a 8x8x8 k-point
Have you check your k-point convergence with the total energy?
> with 180 Ry cutoff for wavefunction.
It is very large for USPP and check it for both pseudopotential
> I've opened the structure in Xcrysden and it looks OK, no weird things.
> *David Furman*
> Pw_forum mailing list
> Pw_forum at pwscf.org
Thanks and Regards
Bramha Prasad Pandey
Indian School of Mines(ISM)
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