Is the Force in Spin spirals calculation correct?

[wdovew]

Dear, VASP Expert,

Thank you very much for your reading and answers. The problems are follows:

When calculating spin-spiral tates, I use two steps of calculation,
1: Non-magnetic calculation of charge density and wave function;
2: The second part reads the wave function and charge density of the previous step, and spinspiral calculation is added to the input related parameters.
After these two-step calculation, I hope to extract the Force file, but I am not sure whether spin-spiral's contribution is considered when calculating the atomic force.
Simply put, can VASP directly calculate the phonon under spin-spiral states? I think that as long as the spin spirals exists, the force on the atom can be calculated reasonably in VASP. I'm not sure if this is correct? Can the Force be calculated in the spin-spirals calculation correctly.

Looking forward to your reply.

Best wishes.

User, wdovew

[martin.schlipf]

From your question it was not exactly clear how you want to calculate spin spirals in VASP. There are two methods, you can do this
1) You can use a supercell approach and actually calculate tilted magnetic moments setting LNONCOLLINEAR = TRUE and the appropriate MAGMOM. This is fully compatible with forces, but your cells might get quite large if you want to calculate phonons.
2) You can use the generalized Bloch theorem and treat the spin-spiral in the primitive cell. Here you need to consider some caveats that are outlined in more detail in the link. But if these are acceptable for your system, you can combine it to obtain forces as well.

One final comment: be careful when combining spin spirals and phonons. If the energy scales of phonons and spin spirals are very different, then it may be advisable to treat the systems separately. And if you are interested in the interplay of phonons and spin spirals when the energy scales are comparable, you may need to go beyond the independent particle picture.

[wdovew]

Dear martin.schlipf,
Thank you very much for your reply.

In order to explore the influence of a Spin Spirals(SS) state on the phonon spectrum, I consider combining the SS state and the finite displacement method to calculate the phonon spectrum.
The second method 2) I used when calculating the influence of a SS state on the phonon spectrum.
In order to save calculation time in Multi-magnetic atomic materials，two steps are performed in VASP: the first step is to self-consistently calculate the charge density: CHGCAR and the wave function WAVECAR of the non-magnetic unit cell; the second step is to read in the non-magnetic self-consistent CHGCAR and WAVECAR, and the non-self-consistent calculation of the atomic force. Add calculation parameters the second step :
LNONCOLLINEAR = .TRUE. ; MAGMOM = ... ; LSPIRAL = .TRUE. ; QSPIRAL =..; LASPH = ; ISPIN =2;

Are such calculation steps reasonable ? A reasonable force constant can be obtained with SS calculation, I think a lot of testing is necessary?

Best wishes.

[martin.schlipf]

Usually the finite difference implementation of phonons (IBRION=5,6) is very robust. So I don't see any fundamental issue here. Just be careful to switch off the symmetry of the system.

It is always a good idea to do some testing afterwards to see if the results make sense. You could for example pick some phonon modes and displace along the eigenvectors and see if the width of the parabola corresponds to the phonon frequency.