In section 6.64 of the VASP manual says it says "For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size L of the supercell." It then goes on to talk about how the tag IDIPOL can be used to calculate the monopole/dipole and quadrupole corrections to the energy.
Based off my interpretation of the above statements, I should use the IDIPOL tag when I want to minimize the interaction of the periodic images of the super cell. For example I SHOULD use IDIPOL if I want to model isolated molecules, isolated slabs, or an isolated charged defect in a bulk system, but I SHOULD NOT use IDIPOL if I want to model a specific density of molecular gas or a specific density of charged defects in a bulk system (i.e. I want interaction between periodic images). Similarly for LDIPOL and LMONO.
Is my understanding correct or no? Any insight would be appreciated, thank you!
P.S.
Does anyone know what value of EPSILON to use for slab calculations? The manual says to set EPSILON = 1 for molecules, and use the bulk value for bulk calculations, but does not specify for slabs.
Hello Erik Nykwest,
I think your understanding is correct. The IDIPOL and LDIPOL flags are used to kill unphysical interactions: in all directions for isolated molecules, in the direction perpendiuclar to the surface for slabs (an ideal surface is a 2D periodic surface, not 3D). In prinicple you could manage without these keyword using large enough supercells, but it happens to slow down the calculations in some cases.
If you need these interactions to be takin into account, do not use the IDIPOL and LDIPOL flags.
With the LMONO and EPSILON flags I am unfortunately not familiar.
I hope it'll help a bit!
Thank you Romain, your answer gives me more confidence in my research procedure. As for the EPSILON flag I asked one of my fellow grad students that models materials with applied electric fields and they said that for slabs and molecules it should be left at the default value of 1 as the thing you are modeling is surrounded by vacuum.
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