[QE-users] Bader Charges for a Spin-Polarized System

[Victor Bermudez]

Happy New Year All, 

         I'm trying to compute Bader charges for a paramagnetic molecule (NO2) adsorbed on MoS2. Normally one would use the PAW wavefunction of the relaxed system as input to pp.x with plot_num=17 to obtain the electron density, which would then be processed with the Henkelman code (http://theory.cm.utexas.edu/henkelman/code/bader/) to get the Bader charges. Unfortunately, pp.x with plot_num=17 doesn't work for spin-unrestricted systems in Quantum Espresso version 6.4.1. The latest release notes say that this has been fixed in vers. 6.5, but I don't yet have access to this most-recent version. I also tried plot_num=21 in vers. 6.4.1, and that doesn't work either. 
            Out of desperation I tried plot_num=0, which according to the pp.x input write-up produces "electron (pseudo-)charge density". That does work, and the results appear reasonable. The right total number of electrons is obtained, and all the atom charges look OK (Mo = +1.18, S = -0.59, N = +0.70, O = -0.37). The MoS2 is essentially charge-neutral and a very small negative charge appears on the electron-acceptor NO2. 
            With that as background, my question is whether or not the use of the plot_num=0 density in this way is in fact valid. I'm not sure what the term "electron (pseudo-)charge density" actually means. Another disturbing point is that the atom volumes found by the Bader code are slightly different for S atoms that are related by symmetry (i.e., by reflection in a mirror plane), even in the absence of the adsorbed molecule. In one S layer the volumes are in the range of 833-841 bohr^3 while in the symmetrically-equivalent layer the range is 821-828 bohr^3. I should mention that I'm using a dipole correction layer (3 Angstroms wide) in the middle of the vacuum space (26 Angstroms wide) because of the finite molecular dipole. I'm assuming that this won't affect the Bader-charge calculation. 
            Any expert advice would be much appreciated. 

Best Wishes,
Vic Bermudez
(US Naval Research Lab. - retired)