How much vacuum should be applied in the HOMO LUMO calculation. I think that choose the ibrav=1 for cubic symmetry in scf code/scf calculation
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Dear Merve Özcan ,
Do you want to perform scf calculation in Quantum Espresso? ibrav=1 mans simple cubic structure. If you have molecule and you want to calculate HOMO and LUMO states, I recommend to create simple cubic as wide as you can calcualte (by means of your computation power) and put the molecule middle of the simple cubic. We use from vacuum to avoid from interactions of neigbour periodic images in order to calculate isolated molecules.
If you want to calculate the surface slab of one of the structure, you have to find coverged vacuum size. For that it is necessary to calculate surface energy for difference size of vacum then you can choose converged vacum size.
Jasurbek Gulomov
Jasurbek Gulomov Erman Salih Istifli many thanks for your reply. I will chose ibrav=1 (cubic symmetry) in scf code as you suggested. Maybe a=20 or 30 Angstrom (vacuum) can enough to avoid from interactions!
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A way to determine whether the vacuum space you incorporated is enough is to calculate the planar electrostatic potential (i.e., the electrostatic potential projected in 1 direction). If the planar potentials have a constant value (flat) beyond where the molecule is located it means that the system is well-contained. however, if the potential increases or decreases it is an indication of neighboring interactions and can affect the calculations. With that aside i think a vacuum of 20 is very sufficient if you have a small molecule. in your choice, you must consider the molecule's size and whether it is polar or non-polar.
Regards.
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