VASP Forum

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

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,

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

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:

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,

Hi Max,

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

Cheers,
Patrizio