Getting different total magnetic moment in 'scf' and 'vc-relax' calculations in Quantum ESPRESSO

Question

I am performing a spin polarized DFT calculations in Quantum Espresso on a Co2MnSi ferromagnetic crystal. For this I have performed these steps:
1) Performed vc-relax calculation to find its lattice constant. Here is some lines from output file:

CELL_PARAMETERS (alat= 10.00000000)
-0.531670336   0.000000000   0.531670336
 0.000000000   0.531670336   0.531670336
-0.531670336   0.531670336  -0.000000000

ATOMIC_POSITIONS (alat)
Co       0.000000000   0.000000000  -0.000000000
Mn       0.265835168   0.265835168   0.265835168
Co       0.531670336   0.531670336   0.531670336
Si      -0.265835168  -0.265835168  -0.265835168
End final coordinates

Lattice constant found is as, a = (10/0.5)*0.531670336 = 10.63340672 Bohr and the final 'total magnetization' found at the end of the calculation is as follows:

  total magnetization       =     5.00 Bohr mag/cell
  absolute magnetization    =     5.39 Bohr mag/cell/cell

2) Now I have plugged the value of lattice constant, (celldm(1)=10.63340672) in scf input file and performed the 'scf' calculation. But in the output file of scf, final total magnetization is different than the vc-relax output.

total magnetization       =     4.56 Bohr mag/cell
absolute magnetization    =     5.01 Bohr mag/cell

My Question:
Correct 'total magnetization' for this system is exact 5.00 (which I have got in the vc-relax calculation) but why I'm getting different total magnetization from the scf calculation? Shouldn't it be same cause this will affect the DOS calculation also? Also the total energy from two different calculation is different. Why is so? Am I performing the steps wrongly?

Please clarify it.
Thank you!

You can find the input and output files here

Answer 1

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 things:

1. Run a standalone scf calculation with the starting magnetizations used in the final scf step of the vc-relax calculation.
2. Do another vc-relax starting from your relaxed cell parameters. You can see in your separate scf calculation that the cell stress has increased a lot. You may converge to a different geometry and ground state than your first vc-relax. You can either compare the energies from the two calculations and choose the lower, or choose one based on the magnetizations on the atoms (if applicable), i.e. whether one is closer to the spin arrangement you expect or desire to study.

Hope this helps.

Answer 2

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 relaxed, including atomic positions and cell volume. You then took the optimized lattice constants, but not the optimized atomic positions, for the SCF calculation. This means the SCF calculation is not the local minimum energy structure you found with the geometry optimization. You should not necessarily expect the magnetic moments to be the same. In this case, I imagine your structure in the SCF calculation has a fairly high degree of stress on it, and the fact that it's not quite a local minimum in the potential energy surface is why you are getting a close (but not identical, and non-integer) net magnetic moment. Regardless, this also explains why your energies are different – the structure isn't the same!

If you aren't happy about the relaxed atomic positions, then you need to ask yourself why. You can't just neglect to update them because they don't look nice. Perhaps you want to enforce a given symmetry. Perhaps there is something missing from the structure that stabilizes the geometry. Perhaps the geometry you think you should have is not actually real at all! These are more problem-specific questions, not a DFT question at this point.