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12

'vc-relax' and 'relax' are two quite different functionalities of the PW module. 'vc' stands for 'variable cell', so the cell shape (angles and length) as well as the atomic coordinates are optimized. Both the forces and stress tensor are calculated in 'vc-relax'. 'relax' only does the atomic coordinates optimization, i.e., only the forces are calculated. ...

11

Should I turn the ISYM = 0 to also change the shape of the unit cell during the structure relaxation? In essence, you are asking how to build a slab model. One special point is that you should add a large vacuum layer (here I assume that direction is along [001]). The relaxation of the slab model is limited to (100) and (010), so you can't change the shape. ...

9

I would not recommend applying external hydrostatic pressure by changing the lattice parameter by hand. This is because fixing the lattice parameters by hand may lead to the incorrect structure. For example, the system may have a pressure-induced structural phase transition that may change it from cubic to tetragonal, and by fixing the lattice parameters by ...

8

Edit: This no longer seems to be the issue, as the atomic positions are apparently essentially unchanged, per @KevinJ.M.'s comment. I will nonetheless keep the answer here because it is still important to consider going forward. It looks like you are getting a different magnetization because the structures are not identical. Your first structure is fully ...

8

Looking at your input and output files, I think the most likely explanation is a different local minimum in the energy, as mentioned by Andrew in the comments. If you look at the final scf step in the vc-relax output, it uses the starting magnetization from the last bfgs step, not the initial values you used at the start of the vc-relax. I would try two ...

7

It is a bit difficult to answer this question, due to the information provided. If you are a beginner, using vc-relax will have its pros and cons. With regards to using nspin=2: Recall that when you use nspin=2, you are saying that you will define the initial magnetization for the involved species. So, just keep in mind that spin-polarized calculations will ...

7

Please read the pw.x documentation on the Bravais lattice index (ibrav). The cell vectors for the primitive cell corresponding to the cubic face-centered Bravais lattice (ibrav=2) are v1 = (a/2)(-1,0,1), v2 = (a/2)(0,1,1), v3 = (a/2)(-1,1,0) The cell volume is $V=|v_1 \times v_2 \cdot v_3| = (a/2)^32=328.85\ \mathrm{a.u.}^3$

6

You can start with this bash script for Aluminum to begin Note: Make sure to fix two quantities while changing third one among INCAR,KPOINTS,POSCAR Lattice optimization for i in seq -w 4.01 0.01 4.05 # change the range needed do cat <<EOF >POSCAR Al bulk FCC $i 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 1 direct 0.0 0.0 0.0 EOF mkdir$i cp INCAR \$i/ cp ...

5

In your case, the crystallographic axes look to be equivalent, so just use 'relax' to start with. This will fix the lattice constants (equivalent to applying pressure) but will move the atoms around - make sure you modify the lattice parameter to reflect the 5% strain, 10% strain etc. Then, perform your SCF calculation. If your crystallographic axes are ...

4

If the system is magnetic you need to use nspin=2. You can use any pseudopotential, unless you want to see SOC, in which case you need to use a fully relativistic pseudopotential.

3

Prior to any production calculation, the convergence study with ecutwfc, k-points and ecutrho should be enough. But, the better should be to do a convergence study using the property you are interested in. Please, take a look in previous discussion here, here and here.

2

There are many reasons behind not converging. Sometimes structure doesn't converge if you describe them wrong. So, in my opinion, the best way to converge a structure is to study it clearly. Moreover, this convergence also varies for observing different properties too. One common trick is to check smearing in your system. For your system check if it ...

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