external pressure

[lsam]

Dear VASP users,

I am doing calculations with 64 atoms TiO supercells. To determine how the occupancies are set I use the Methfessel Paxton (ISMEAR=1) during the relaxation and the tetrahedron method with Blockl corrections (ISMEAR=-5) to calculte the total energy.

Which value of the external pressure should I take in account to evaluate the convergence of my results? (from the relaxation or from the total energy calcukations ?)

I obtained to the external pressure 0.75 kB in the end of the relaxation and -4.07 in the total energy calculation. Is this supercell converged?

Thanks for you help.

Best Regards

[admin]

As the total energies calculated with the Blöchl method are more accurate, the pressure calcuated with ISMEAR=-5 is more reliable, too. I would suggest to do the following if you need very precise results:
1) check if the electronic structure of your system has a gap. If it does, add a few (2-3) more ionic relaxation steps using the Blöchl method untill the pressure is lower than 1kB.
2) if it does not, please do 2-3 more static steps with a slightly changed lattice constant (a few mÃ…) using ISMEAR=-5; and then fit the E/lattice volume parabola to find the equilibrium volume.

[lsam]

The lattice parameter of my cell can change something like 0.4 mA. But I still not understandig the following:

I did full relaxation of my TiO cell with 64 atoms with

IBRION=2;
ISIF=3;
NW=100;
ISMEAR=1;

and the external pressure was 0.54 kB.

Then I calculate the external pressure and the charge density with

IBRION = -1
ICHARG = 2
ISMEAR = -5
NSW=0

and the external pressure was -4.07 kB

Finnaly using the calculated charge density, increasing the number of k-points and bands, I calculate the optical properties and DOS

IBRION = -1
ICHARG = 11
ISMEAR = -5
NSW = 0

and the external pressure was 7.65 kB

I did convergence tests, studied the Ecut as function of the external pressure and the k-points with the total energy.

I'am doing something wrong? Or can I ingnore the oscillation of the external pressure?

Best Regards,

[centauris]

If I remember correctly, TiO is a wide band gap semiconductor. You should NOT use first order MP to integrate near fermi surface. your relaxation result might not be correct, so is the static calculation with improved Bolchol integration.