Generating PARCHG

Quick Description:  Partial charge density  is (mathematically) the squared eigenstate for a given k and eigenvalue. For a given band, this is essentially the same as the local density of states. In its final form, the PARCHG file will have units of electrons (charge density * the volume of the unit cell). Summing up the local density of states for the eigenvalues specified at the corresponding band will give the band-decomposed partial charge density. Visually, this allows you to see clouds (isosurfaces) of charge that fall into a specified energy range. If done correctly, the top view of this should be fairly similar (but sharper and cleaner) to what you view via STM.

(example of don't-remember-what-but-it-gives-you-an-idea)

The Point: You need this file to do simulated STS and STM.

Prerequisites: An optimized job with a preconverged WAVECAR. You will need to resubmit, and you may consider creating a new folder for this job using the new atomic coordinates (so change CONTCAR to POSCAR).

Notes: This is a very fast calculation, so the majority of your work has already been done. For more information, check here.

You'll need to add some more lines to your INCAR file, and in particular you care about setting the following lines (which you can just copy/paste into your file):

ISTART = 1
LWAVE=.FALSE.
LPARD=.TRUE.
LCHARG=.TRUE.
EINT=-1 0
NBMOD=-3 

Once you have set these, you're ready to submit a job as you normally would.

In the case shown above, the EINT setting specifies to include electrons who energies ranges from -1 V up to the Fermi energy. The negative sign indicates that you are looking at occupied states of the system (and for unoccupied, you would use a positive number). The NBMOD setting specifies that the Fermi energy is treated as being 0 in energy. These energies refer to the voltage bias that you would have been scanning at, and it is common to use a higher (in absolute magnitude) bias.  In this case, the presumption of the bias would be -1 V, a fairly common parameter, and so the treat the tip as sampling electrons in the range from -1 V to 0 V.

Alternatively, if you specifically want certain "bands", indexed the same way they are in the EIGENVAL file, you would use:

LPARD=.TRUE.  #calculate partial charge density
IBAND= 1 2 3 5 10 20    #indexes of bands to be included in partial charge density

For your specific case you would change EINT or IBAND according to what you need.  And, if you look at the manual link above, you'll see there are other variations as well.

A caveat from Jon: "Now, all that said... there is one additionally confusing thing which I think people generally ignore.  The energies that come out of DFT are the "Kohn-Sham energies" which don't necessarily match the real-life electron energies... though people have found you can re-scale them to match correctly.  We always ignore that (we don't rescale or anything... and without having more information there really isn't a way to do so) - so it's just an extra tidbit of knowledge that you can't really do anything about and it can just sit in the back of your mind and nag at you."