Hello Mr. Gruszka,

If you are going to do an optimization of a crystal, I should advice you to relax all the cell  parameters (so the keyword "vc-relax"). This because I suppose you're starting from crystallographic data, that are calculated at low but non-zero Temperature. Instead, a standard DFT calculation is done at 0K. So if you let the system to relax also the cell parameters, it could happen that you will reach a geometry with low energy.

Different is the case if you are studying a surface. If you put a layer of vacuum, it could be useful to keep the volume constant.

Moreover you could then compare your results (doped and undoped system), because they are done in the same conditions (same relaxation conditions).

I hope I have been useful,

Best regards.

Giovanni

Hello Mr. Gruszka,

If you are going to do an optimization of a crystal, I should advice you to relax all the cell  parameters (so the keyword "vc-relax"). This because I suppose you're starting from crystallographic data, that are calculated at low but non-zero Temperature. Instead, a standard DFT calculation is done at 0K. So if you let the system to relax also the cell parameters, it could happen that you will reach a geometry with low energy.

Different is the case if you are studying a surface. If you put a layer of vacuum, it could be useful to keep the volume constant.

Moreover you could then compare your results (doped and undoped system), because they are done in the same conditions (same relaxation conditions).

I hope I have been useful,

Best regards.

Giovanni

If you do not want to directly compare to some experimental finding of lattice constants with the dopant, then I would definitely use vc-relax to modify cell parameters.  The dopant will have a sizable effect on the forces (probably) and you want to take this into account when finding the ground state energy.

The only caveat to this is if you want to fix at some experimental constants because you don't think your pseudopotential will give accurate results.

From my point of view, you just need optimize atomic positions of the doped system if your purpose is to investigate the electronic structure of the defect. You are advised to first fully relax the cell (cell parameters+atomic position) to its ground state, and then relax the doped system while keeping the cell parameters unchanged. The underlying assumption is that in the  low concentration (only isolated defect is our concern ) limit, the crystal structure should keep unchanged.

by the way, QE is a code for first principle calculation where crystal structure infos (even imaginary ones)must be provided first, other than to predict unknown crystal structures.So, you have to obtain reasonable structure from literature. 

First of all, you should consider an approximate dimensions for the hexagonal Unit cell based on the geometrical principles, bond lengths and bond angles. You can find some information about your structure through using the results of other numerical methods. In order to optimize your structure in Quantum ESPRESSO code, you should use vc-relax input code. 

Thank You all for your answers.

I can see now, that this case  is more of my assumptions than 'only one good path' that I should follow..

Ang Feng: I can imagine that when only isolated defect is present, the latter will "arrange" to fit this inclusion, so relaxation of whole cell is needed .  I think that also a much bigger supercell is appropriate. 

I have one simple question, I have performed vc-relax calculation of doped system with out spin-polarization. Is it okay to use same relaxed coordinates and lattice parameters for spin polarized calculations.?

Well, magnetism strongly affects the structure, therefore I would perform another relaxation (or vc-relaxation in this case). Also, when You do this step You will gain additional informations and can check this on your own skin.

Thanks for the answer. I performed the vc-relax calculations, not that drastic change in lattice parameter about 0.01 A.

Dear All,

I am new to QE. I want to calculate high pressure with different volume. So I want to ask my two queries

(1) For changing volume we will change lattice constants or we can change volume in QE by some parameter and it automatic will change lattice constants.

(2) After getting internal energy at different volume corresponding actual pressure QE gives in OUTPUT file or by fitting E-V data we calculate pressure corresponding to volume.

Thank You All

Shilendra

if you are using 2D system it is better to use 'relax' instead of using 'vc-relax'

Depending on what type of simulation you are using, you will get different physics. Neither is wrong and both can give you information. If you use 'vc-relax', you are simulating the effect of periodic defects on the lattice constants of the entire crystal, as well as on the local atomic positions of the atoms. On the other hand, 'relax' implies that the lattice constants are fixed to the values set by the rest of the crystal (implying that the rest of the crystal is more or less defect-free). Performing vc-relax with and without a defect will also give you information about the effect of the defect on the lattice constant.