Steps in case of ICHARG=11 in VASP

Question

I am confused regarding the steps when we use ICHARG=11. This is used to plot the band structure. This means we are starting the calculation from a pre-converged CHGCAR file and keeping the density fixed throughout the calculation. If we know the density, we can calculate the potential (at least upto GGA level), and after solving the KS equation in one step, we can get the eigenvalue at any k-point we want. But, when running with ICHARG=11, there are some scf like steps in the output. From the VASP manual, "charge density will be kept constant during the entire electronic minimization". What is the meaning of electronic minimization if we keep the density fixed?

Answer 1

This VASP tutorial contains what you're looking for. Let's look at their stdout.

 entering main loop
       N       E                     dE             d eps       ncg     rms          rms(c)
DAV:   1     0.384469864424E+02    0.38447E+02   -0.96726E+02    16   0.293E+02
DAV:   2     0.345967752349E+01   -0.34987E+02   -0.34942E+02    32   0.450E+01
DAV:   3    -0.244421465184E+00   -0.37041E+01   -0.34307E+01    16   0.308E+01
DAV:   4    -0.312489880281E+00   -0.68068E-01   -0.66911E-01    16   0.508E+00
DAV:   5    -0.313453098582E+00   -0.96322E-03   -0.96305E-03    32   0.506E-01    0.307E-01
DAV:   6    -0.314470890869E+00   -0.10178E-02   -0.17849E-03    16   0.332E-01    0.155E-01
DAV:   7    -0.314569344422E+00   -0.98454E-04   -0.23774E-04    16   0.137E-01
   1 F= -.31456934E+00 E0= -.16030702E+00  d E =-.308525E+00
 writing wavefunctions

They then explain below that segment that

The initial electronic charge density is read from the POTCAR file and remains fixed for the first 4 steps. This is because the Hamiltonian is iteratively diagonalized and, thus, the KS orbitals used to update the charge density should only be trusted after a warm-up period. Then, the charge density is updated before entering the next iteration step, i.e., not in the final iteration step. See rms(c) column.

To answer your question, optimizing the Kohn-Sham orbitals (even with a fixed electron density) is an iterative process, so that's what you're likely seeing during your ICHARG=11 calculations, specifically, the rms column:

• rms - weighted absolute value of the residual vector of the KS orbitals
• rms(c) - absolute value of the charge density residual vector