Author Topic: Inter-comparison between two different supercells having large and small size  (Read 161 times)

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Offline IndranilM

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Hi,
Can you please help me with one query?
I want to know how the binding energy (adsorption energy) varies with 'supercell size/dimensionality'.
For example, I have two supercells (3*3*1) and (6*6*1) and I want to calculate the binding energy of one gas molecule (e.g physisorption of hydrogen/CO) in a similar substrate material (e.g. graphene) for both cases (viz. large supercell and small supercell of the same material, graphene). Will there any significant difference in binding energy between them? or for both the cases of small supercell and large supercell ((3*3*1) and (6*6*1)), the binding energy will be nearly equal?

Offline filipr

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When an atom/molecule is adsorbed on a surface it alters the electronic environment in the vicinity of the adsorbption site. The effect of this will be a change in the electronic density and thus a change in the potential from the charge distribution. When you go far enough away from the adsorbtion site one will expect the local density and potential to look like the pristine interface - but how you have to go away depends on how the material responds to the adsorbant. There will be multiple effects that determine this: how much the geometry of the interface atoms actually change (nearby atoms will get pushed/pulled) and how much the electrons screen the adsorbant (dielectric properties). Depending on the material, these effects can be short range or long range.

When you do a supercell calculation, it is still a periodic crystal, i.e. you repeat the adsorbant every 3 or every 6 or so unit cells. So you create artificial system with repeated adsorbants with a density/concentration of adsorbants which is typically higher that the system you are trying to model. If you want to model a single isolated absorbant you have to make sure that the absorbants can safely be regarded as isolated, i.e. that the distance between them is longer than the above described effects. The only way to ensure that is to converge adsorbtion energy with respect to the supercell size. So you have to do adsorbtion energy calculations of increasing supercell sizes, e.g. 3x3, 4x4, 5x5, 6x6, 7x7, 8x8, ..., until the energy changes less that some threshold that you consider negligible.

Offline IndranilM

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Thank you so much Filipr for your reply.
May be I'm not clear to you in my earlier query. I'm elaborating this for your further clearance. Please note:
1. I have considered single isolated absorbant (e.g. CO) for each cases
2. I have prepared two supercells of dimensionality (3*3*1) and (6*6*1), separately

I want to know will there any significant difference in binding energy between them? For example, (3*3*1) supercell with single isolated absorbant (at the center position) and (6*6*1) supercell single isolated absorbant (at the center position)
or for both the cases (small supercell and large supercell) , the binding energy of the single isolated absorbant will be nearly equal?


Offline filipr

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Yes, there will be a difference in the adsorbtion energy. Whether the difference is significant is impossible to say. As I said, it depends on the actual system and how big a difference you consider to be "significant". The only real way to find out is to do both calculations.

Offline IndranilM

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Thank you Filipr for your reply.