BEXT coordinates

[patrizio_graziosi]

Hello,
I would like to apply an external magnetic field along a certain directions; I have seen that it is described by three numbers, units in eV, https://www.vasp.at/wiki/index.php/BEXT, but I do not understand if these are internal (unit cell axes) or Cartesian coordinates. Can you please help me in understanding this point, please?
Patrizio

[max_liebetreu]

Hello Patrizio,

Thank you for reaching out to us on the VASP forum!

For spin-polarized computations, BEXT is a scalar parallel or anti-parallel to the spin direction.
For non-collinear computations, BEXT relates to the Pauli matrices, so each component relates to the corresponding entry of SAXIS, which gives the spin quantization axis - given in Cartesian coordinates.

Hope this helps!
Best regards,

[patrizio_graziosi]

Hi Max,
that's great, thanks!
I am doing non-collinear calculation on chiral systems. Because I want to apply a magnetic field along the axis of the chirality-defined helix, I must define SAXIS as pointing in that direction, as per https://www.vasp.at/wiki/index.php/SAXIS, then set BEXT = 0 0 b, where b is the field strength in eV. Is this correct?

Patrizio

[max_liebetreu]

Hi Patrizio,

Yes, that is correct. SAXIS sets \(s_x, s_y, s_z\), which are used to define the direction of the spin quantization axis \(\sigma_3 = \mathbb{s} / \|\mathbb{s}\|\) with respect to Cartesian coordinates. If you set BEXT = 0 0 b, you apply your magnetic field in the direction of \(\mathbb{s}\) with a strength of \(b\), given in eV.

So you write:

Code: Select all

SAXIS = sx sy sz
BEXT = 0 0 b

and simply plug in your values for sx, sy, sz and b.

Does that answer your question?

Best regards,

[patrizio_graziosi]

Hi Max,

Yes, this answers straight my question, thank you so much!

Cheers,
Patrizio