[CP2K-user] electrostatic potential and PDOS plot

Phil G. phil... at aol.com
Thu Jan 2 10:44:31 UTC 2020

Dear CP2K-friends,

regarding to the CP2K manual in the section V_HARTREE_CUBE, the calcutated 
electrostatic potential has an opposite sign than the expected physical 
one. That means for me, that I have to invert the sign of charge in order 
to obtain the physically "correct" potential curves. But as far as I 
notice, the curves or part of lines for the local vacuum region are always 
lower than that of the crystal slab (using 3D periodicity with a certain 
vacuum space along z-axis). That makes no sense, since the potential values 
of vacuum level are always higher than that of the slab due to positive 
work function. Maybe the potential slope must be of opposite direction than 
the expected physical one in the description of this manual section?

As an example, I use single crystal slab LiNbO3 (lithium niobate, 
ferroelectric phase at 273 K with a spontaneous polarization in z-direction 
(polar axis)). The slab has the size of about 10.3 x 8.9 x 31.5 Angstrom 
while the supercell has the length of 100 Angstrom in z-direction (i.e. 
about 68.5 Angstrom vacuum space). After obtaining the V_Hartree_Cube file, 
the calculated electrostatic potential (after using the cube cruncher file 
for converting the cube format into z-dependent potential values) has 
exactly the opposite slope than I expect for the polarization-induced band 
bending on both surfaces of the LiNbO3 slab. That means that the calculated 
potential slope has an opposite direction than that of physical/expected 
one (see espPlot.png file attached here, with dipole correction as a 
potential jump in the vacuum region). 

When I want to obtain PDOS of every slab cuts containing 3 atom layers (Li, 
Nb, O), called as trilayer, and plot over the z-direction, then one can see 
that the shifting of the PDOS plots (e.g. relative to the Fermi level) 
follows the same direction as the calculated slope of the electrostatic 
potential. That means that the electrostatic potential must have exerted an 
influence on the energy value of the PDOS depending on the z-position of 
the corresponding atom layers (see LLPEz1to14wES.png file attached here).

So the question is: Does the physically "wrong" slope of the electrostatic 
potential (either directly or indirectly) affect the calculation of the 
energy values of the PDOS ? 
Or another question: For what reason the convention was made for the 
V_HARTREE_CUBE ? Wouldn't it be better to make the physically correct sign 
choice in the CP2K calculation (charge density, vacuum level, ...) ?

Kind regards,

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