[Pw_forum] Energy keeps going down

[Bramha Pandey]

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
> method.
> 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
> behavior.
> 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
for system.

>
> P.S
> 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
> grid
>
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
convergence .

> I've opened the structure in Xcrysden and it looks OK, no weird things.
>
>
>
> --
>
> *David Furman*
> *
> *
>
>
>
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-- 
Thanks and Regards
Bramha Prasad Pandey
Indian School of Mines(ISM)
Dhanbad, INDIA.
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