MnWO4 supercell stretching in [110] error

[Raghavender A T]

Dear Sir,

I am trying to create a supercell for MnWO4 which stretches out along [110] direction. It has monoclinic structre P2/c with 12 atoms. I used the following command but it gives the error

cif2cell -f MnWO4.cif --supercell=[1,1,0] -p pwscf --pwscf-cartesian-latticevectors --no-reduce --outputfile=MnWO4-110.inp

***Error: Supercell setup: The supercell map must be invertible.

I created Supercells stretching out in [211], [221] and [222] directions but not in [110].

Could you please give your suggestions in creating MnWO4 supercell stretching in [110] direction.

Regards

Raghavender

[Torbjörn Björkman]

The 'supercell' command does not take Miller indices as its input. In the form you give it, it will describe how unit cells are stacked along the lattice vectors. So that if you give the command

--supercell=[2,1,1]

Then that does not give a unit cell that stretches out in the [211] direction, but one that stacks 2 unit cells along the a lattice vector and 1 unit cell along the b and c vectors. So it stretches out the cell along the [100] direction. Please refer to the manual (a pdf file in the Documentation subdirectory of the installation package) to see how supercell generation in cif2cell is meant to work.

The above form for the supercell command is really a shorthand for multiplication by a diagonal supercell matrix. What you want to do is to stack cells along a direction that is not along one of the lattice vectors generated by the program. That requires a supercell map that also has off-diagonal elements (again, read examples in the manual). The easiest way to think about this in your case is that you first want to make a cell that has one of its lattice vectors along the [110] direction. Then you can multiply that by whatever you like. The most obvious way of doing this is to just take:

--supercell=[[1,1,0],[0,1,0],[0,0,1]]

This map gives you three new lattice vectors in terms of integer multiples of the old ones. Now your first lattice vector is along the [110] direction, and you can make it (say) 3 times longer:

--supercell=[[3,3,0],[0,1,0],[0,0,1]]

Visualizing the result will show you that this is a rather awkward-looking cell since you have a rather small angle between the first and second lattice vectors. We could have gotten an angle closer to 90 degrees if we in the first step had chosen our second lattice vector as [-1,1,0], which would instead have given

--supercell=[[3,3,0],[-1,1,0],[0,0,1]]

for a really nicely shaped cell along the [110] direction. But this supercell is twice as large as the previous one, so whether this is good depends on what computations you can afford.

Generally speaking, if you want a cell along the [hkl] direction, and you start from the conventional (principal, 'P', what you get with the 'no-reduce' flag) cell, then you can always use the supercell map:

--supercell=[[h,k,l],[0,1,0],[0,0,1]]

but beware that this may give you a very awkwardly shaped cell.

Hope this helps.
Torbjörn

[Raghavender A T]

Dear Sir,

Thank you for suggestions. It really helped me.

Regards

Raghavender