[Pw_forum] Energy window in STM simulation

[Gabriele Sclauzero]

Dear David,

     strictly speaking what you want (can) simulate with this tools is the 0K limit of your system. The temperature corresponding to the electronic smearing that you introduce is a fictitious one, and one should be careful in interpreting it as a physical T. I agree that in principle one should check convergence of the STM image w.r.t. the smearing (and therefore k-point sampling), as one does for energies or other quantities of interest. In practice, I believe that for most cases the image will be close enough to convergence for standard choices of the smearing. As an option, one could forget the smearing used in the scf calculation and adopt a very small value for the STM image, forgetting about the consistency I've mentioned previously. You could try to perform a small test, it would be useful for everyone. You can either modify the code, or trick it by changing by hand the smearing value in the *.save/data-file.xml file.

Regards,

Gabriele



> Let's say we're simulating a room temperature STM image of a metal or semimetal.  Here's my logic-- please correct me if I'm wrong.  Typical smearing widths of the order of 0.01 Ry correspond to non-negligible populations of excited states (for Fermi-Dirac smearing, 0.01 is equivalent to ~1500K).  It seems to me that when you add in states to the LDOS that are, say, above the Tersoff-Hamann energy window, then you could very well be adding in charge density from states composed of higher-index periodic functions [psi=planewave * periodic function] that really shouldn't contribute much to the LDOS at 300K.  The shapes of these higher-index periodic functions could distort the STM image.  So, to minimize the distortion, you'd want to run the PW calculation at a rather low smearing width (~0.002 Ry), which of course would require a finer k-point mesh.  Does this argument make sense?


§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne







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