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I'm getting my bachelor degree in physics and in my structure of matter course we never talked about bulk, monolayer and so on. I can understand Bravais lattice , reciprocal lattice and so on but I'm very confused about the pratical implementation of bulk structure for Quantum Espresso. For example I think that this code gives me a monolayer becayse I only have 3 atoms in a unitary cell and the c parameter is high enough to eliminate the periodicity:

&SYSTEM
ibrav = 4
a = 3.1401972104
c = 13.3662099134
nat = 3
ntyp = 2
ecutwfc = 70

ATOMIC_SPECIES
Mo 95.9400 Mo.pbesol-spn-kjpaw_psl.1.0.0.UPF
S 32.0660  S.pbesol-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
Mo  0.3333332524 0.6666665101 0.25000000000
S   0.6666666723 0.3333244955 0.13328123440
S   0.6666667238 0.3333332449 0.36667187666

Instead here we have a bilayer because there are 6 atoms:

&SYSTEM
ibrav = 4
a = 3.13616595240
c = 12.0411134300
nat = 6
ntyp = 2
ecutwfc = 70

ATOMIC_SPECIES
Mo 95.9400 Mo.pbesol-spn-kjpaw_psl.1.0.0.UPF
S 32.0660  S.pbesol-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
Mo  0.3333333000 0.6666666700 0.25000000000
Mo  0.6666666700 0.3333333000 0.75000000000
S   0.6666666700 0.3333333000 0.37563781800
S   0.3333333000 0.6666666700 0.87563781800
S   0.6666666700 0.3333333330 0.12436218100
S   0.3333333300 0.6666666670 0.62436218100

But my advisor told me that this is the bulk structure , not the bi-layer. What am I doing wrong? I'm sorry for the really newbie questions.

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1 Answer 1

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Indeed, this input represents a bulk structure

The trick here is that the input is using fractional coordinates:

ATOMIC_POSITIONS (crystal)
Mo  0.3333332524 0.6666665101 0.25000000000
S   0.6666666723 0.3333244955 0.13328123440
S   0.6666667238 0.3333332449 0.36667187666

So, the final Cartesian coordinates will always be proportional to the cell parameters: if you increase the cell parameters, the positions will increase in absolute value too. In this case (assuming $b=a$ and cell parameters in Angstrom), the Cartesian coordinates will be:

Mo  1.046732149 2.093464315 3.341552478
S   2.093464825 1.046704651 1.781464957
S   2.093464986 1.046732126 4.901013273

Now, if you use them, you can change the cell parameters without moving the atoms too (probably you need to change the type of coordinates in the input from fractional to Cartesian).

About the second input, no, it is no a bilayer because, as there are fractional coordinates, you have more atoms in the same cell.

I strongly recommend you to use some software to produce your monolayers (or other supercells). For example CIF2Cell.

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  • $\begingroup$ I really really thank you. Why if the coordinates of my atoms are linked to cell parameters than I'll have a bulk structure? If they're not then I'll have a monolayer? Sorry for the silly questions but I have just a month for complete my thesis $\endgroup$ Jul 17, 2023 at 15:07

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