Dear developers,
I am interested in the adsorption of polycyclic conjugated hydrocarbons on metal surfaces and the influence of this interaction on their aromaticity.
I was therefore pleased to see that, starting with version 6.6.0, it is possible to calculate NICS values using periodic boundary conditions.
I subsequently began testing how this method could be applied to my research questions.
For molecules in the gas phase, the approach appears very promising, as I am able to reproduce values obtained with molecular quantum chemistry codes.
However, I quickly noted that the calculation of NICS values (or σ, because NICS = -σ) of an adsorbed molecule is highly sensitive to the chosen smearing parameters.
(Note that, in general, SIGMA needs to be small and ISMEAR cannot be >0 in order to converge the NMR calculation)
Particularly, I do not observe meaningful convergence upon decreasing SIGMA for different ISMEAR settings. Instead, the resulting values appear rather arbitrary.
Some values as an example: σ_zz(0) values of Azulene, 2 Å above a Cu(111) surface
- ISMEAR=0 & SIGMA=0.01 : -83.3 & -105.9 ppm
- ISMEAR=0 & SIGMA=0.001 : -9.5 & 50.8 ppm
- ISMEAR=-1 & SIGMA=0.01 : 38.7 & -1.8 ppm
- ISMEAR=-1 & SIGMA=0.001 : -154.8 & -73.9 ppm
Furthermore, I observed in additional test calculations that NICS values (or σ) above "diagonal" and "parallel" bridge positions are not equivalent, although they should be identical according to the threefold symmetry of the (111) surface.
Following that observation, I repeated the same Azulene adsorption calculation as before, but rotated the unit cell by 60°.
This changes the orientation of the surface and molecule with respect to the x- and y-axes, while leaving the local adsorption geometry unchanged.
I had assumed that the GIPAW approach ensures gauge independence in NMR calculations, so that such a rotation of the unit cell should not affect the results.
Therefore, the σ_zz(0) values would be expected to remain identical. But they are not:
- ISMEAR=0 & SIGMA=0.01 : -83.3 & -105.9 ppm
- ISMEAR=0 & SIGMA=0.01 & rotated cell : -57.8 & -106.3 ppm
Long story short - I have two main questions:
1. Is there any experience with calculating NICS values (or σ) for molecules on metal surfaces, in particular regarding the appropriate choice of smearing parameters? Is this meaningful from a methodological standpoint at all?
2. Why are there different NICS values (or σ) upon rotating the cell? Should these quantities not be invariant under such transformations?
Thanks a lot!
Jakob