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I am performing a transition metal slab calculation using norm conserving PPs.
For some reason or the other, the calculation seems to converge much faster for iscf=17 than for iscf=7. When using iscf=17, however, I get warnings about the density going negative. Taking a look at the code, it says:

!! Note that, ignoring model core charge, as long as boxcut>=2
!! the shifted density is derivable from the square of a Fourier
!! interpolated charge density => CANNOT go < 0.
...
!! Fourier interpolated core charge can go < 0 due to Gibbs
!! oscillations;...

Does this mean that the negative densities could arise from the fact that I have a pseudo-core? How come this only happens for iscf=17?
Ecut and boxcut should be OK. Any good suggestions what to do otherwise to prevent this behaviour?

Hi,

Since you have a slab, I assume you have a lot of vacuum in your unit cell. The negative densities are caused because of this vacuum (Fourier transformation of the localized boundary between material and vacuum needs an infinite basis to be described exact, cfr that complain about Gibbs oscillations), and instead of describing the vacuum by a zero electronic density, the electronic density can be numerically a (small) negative density.

These warnings either aren't triggered in different algorithms (because the convergence runs differently) or because they just aren't implemented.
When your calculation aborts of this warning you can play with other electronic convergence parameters (I suggest looking into iprcel between 41 and 49 if you haven't implemented this already).

with kind regards,

Annelinde