I understand graphene has a hexagonal structure(ibrav=4), but in this example,
https://www.afs.enea.it/buonocor/graphene-with-pwscf.html
they choose ibrav=12 (while running "vc-relax", "scf", "nscf", "bands" calculations), ibrav=o (while running "relax" calculations).
Is this still correct? Why did they use ibrav=0, 12?
Yes this is correct. In this code they created a alternative way for building graphene. We can create hexagonal structure using monoclinic bravais-lattice. And for relax free bravais-lattice used for free movement of each atom which can be the key for finding most stable position.
Thank you Chayan, What about when we are doing the "scf", "nscf", "bands" calculations, are we not supposed to set ibrav=4(Hexagonal)?
well, you can use both ibrav=4 or 12. but when you will use ibrav=12 you must have to set the angle between a and b about 120 degree by setting cosab=-.5. So by setting the angle 120 degree you can get the same lattice structure as hexagonal lattice.
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