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Dear VASP users and Experts

I have two questions. I wonder if any of you can explain to me how to do a correct comparison of the band structures for a system containing

A) The (pure) surface
B) The surface + a molecule absorbed on the surface

Both calculations have the same KPOINTS, and the same size of the unitcell (super cell),
with an empty vacuum space for the A case and the molecule is on top of the surface in B. The number of atoms inside the super cell is different.

Another second question is how can I do the projected band structure on the molecule ?

Many thanks in advanced

Oscar RP

I am having the same questions, as my research involves liquid/solid interfacial interactions. I know that you have to be very careful when choosing the unit cell in the case of B because you do not want any long-range static interactions between the molecules leading to "charging" or the surface. Also, A should be a supercell (~100 or so atoms). The band structure could be compared for the pure surface, the pure molecule, the surface + molecule. I would compare all three, rather than just comparing two. I think the projected band structure would work the same as in A, just setting the LORBIT tag in INCAR. All of these things I will be looking at in the next few months as I am just to the point in my project where interfacial work is to be considered, but we should communicate our progress because I think we will be able to help each other out once we get started. Best of luck!

if you have a system with vacuum, please use the LVTOT input parameter to obtain the potential on a regular grid (file LOCPOT).
For unsymmetric slabs, please use IDIPOL=3 (assumed z is the surface normal)
then use the tool vtotav to obtain the planar averaged potential.
the constant potential in the center of the vacuum can be regarded as the reference point of the energy scale (E=0). please align all KS energies with respect to the respective E-references in order to obtain the band sructures on the same energy scale.

Thanks, that helps answer several of my questions. Where do I obtain the vtotav tool? Also, when you say to allign the KS energies I am assuming you are saying to shift the energies with respect to the reference point?

1) please contact vasp.materialphysik@univie.ac.at for vtotav.f
2) yes