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When I perform Scanning Tunneling Spectroscopy (STS) measurements on Ag(111) to measure the DOS, I find a steep increase of the DOS around -67 mV relative to EF. This is due to the Shockley surface state.

This surface state I want to reproduce using VASP. I made a 7 layer slab and geometrically relaxed it. Afterwards I calculated the DOS with 6001 points between -6 and 6 eV with a SIGMA of 0.01 to have a high energy resolution, but I cannot find an increase in the DOS around -67 mV (or wheresoever). Are there people who have experience calculating the DOS of a Ag(111) slab and retrieving this surface state?

I would procced as follows:
1) have a look at PROCAR (LORBIT=12) and check the character of all possible states at that energy (they should have a charge distribution mainly localised on the surface Ag atom(s).
and/or
2) simulate the corresponding STM image(s) by making ( a series of) partial charge calculations in the energy range -0.067 eV with respect to the Fermi level (eg -0.077 -0.067 and -0.057eV) and have a look at the simulated STM images (LPARD=.true. ; please have a look at the chapter 'band decomposed charge densities' in the online handbook)
these procedures should give you results which are more suitable than the DOS (the surface state peak may not be resolbved properly because of interpolation of the graphic software you use,...)