Question about mn2au unit cell

[Ratthaphong Rangngoen]

Hi everyone,

I am constructing  Mn2Au structure. I found that in mn2au.cpp has unit_cell.dimensions[2] = 3.0; I'm not sure that when I set unit cell size like below material file. Is this correct? because I think from 

 unit_cell.dimensions[0] *= unitcell::internal::unit_cell_size_x;    

unit_cell.dimensions[1] *= unitcell::internal::unit_cell_size_y;    

unit_cell.dimensions[2] *= unitcell::internal::unit_cell_size_z;

the unit cell in z direction will be three times right? Could you please explain this to me ?

Thank you in advance.

#------------------------------------------
# Creation attributes:
#------------------------------------------
dimensions:unit-cell-size-x = 3.328 !A
dimensions:unit-cell-size-y = 3.328 !A
dimensions:unit-cell-size-z = 8.539 !A
dimensions:system-size-x = 5.0 !nm
dimensions:system-size-y = 5.0 !nm
dimensions:system-size-z = 6.85 !nm 

[gabo...@gmail.com]

Hi,

Maybe a developer or another user can confirm, but I suspect mn2au.cpp (1) was designed to use in the input file the line

dimensions:unit-cell-size = 3.33 !A

instead of

dimensions:unit-cell-size-x
dimensions:unit-cell-size-y
dimensions:unit-cell-size-z

Tetragonal Mn2Au (I4/mmm) has lattice constants a = b = 3.33 Å and c = 8.54 Å (2).

The c/a ratio is:

c/a = 8.54 Å/3.33 Å ≅ 2.56 ≈ 3

Rounding 2.56 to 3.0 might be where that comes from for:

unit_cell.dimensions[2] = 3.0

If you don't like the rounding error associated with that, without modifying mn2au.cpp, you could probably use:

dimensions:unit-cell-size-x = 3.33 !A
dimensions:unit-cell-size-y = 3.33 !A
dimensions:unit-cell-size-z = 2.85 !A

where dimensions:unit-cell-size-z is 8.54 Å / 3 ≅  2.85 Å.

Of note, Nd2Fe14B has lattice constants a = b = 8.8 Å and c = 12.2 Å (3).

In nd2fe14b.cpp (4), I see the line:

unit_cell.dimensions[2] = 12.2 / 8.8;

Alternatively, you should be able to address the rounding error by modifying mn2au.cpp.  For example, you could probably modify mn2au.cpp to be similar to nd2fe14b.cpp by changing "unit_cell.dimensions[2] = 3.0" to:

unit_cell.dimensions[2] = 8.54 / 3.33;

Hope that can help.

(1) https://github.com/richard-evans/vampire/blob/main/src/unitcell/mn2au.cpp

(2) https://arxiv.org/abs/2404.15525v1

(3) https://doi.org/10.1103/PhysRevMaterials.2.074410

(4) https://github.com/richard-evans/vampire/blob/main/src/unitcell/nd2fe14b.cpp

nd2fe14b.cpp

Kind Regards,

Gavin

VAMPIRE user

[Ratthaphong Rangngoen]

Thank you very much for your response. I will try it.

Kind Regards,

Book

On Tue, Oct 14, 2025 at 11:45 AM Ratthaphong Rangngoen <65010...@msu.ac.th> wrote:

Thank you for your response. I will try it.

Kind Regards,

Book

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