Analysis "only when with core charges" yields unexpected results

[tp.joe]

Hello Prof. Henkelman,

I am dealing with charge analysis of an oxygenated aromatic compound adsorbing on Ni surface, using VASP 5.4.1 with optB88-vdW for dispersion force correction.

In short, the issue I encountered with is that when I included core electrons, the carbon atoms in the ring become positively charged and hydrogens become negatively charged (ca. -0.2 to 0.4 per atom). It is very counter-intuitive to see electron transfer from a metal surface to aromatics but having hydrogen as the sink instead of the carbon ring. In contrast, if I don't include core electrons the results become as expected, i.e. carbons in the ring are negatively charged, while hydrogen is almost neutral. The results by both including and not including core charges are attached.

Note that weight function was not used (I tried, but the system crashed probably due to large CHGCAR file size), but I doubt if this would critically affect the result.

My question is that in such system (CONTCARand INCAR attached), could there be anything tricky that I should be aware of, which could otherwise compromise the analysis? because I just could not imagine hydrogen being significantly negative. Or is weight function is indispensable when doing analysis using core charges?

Thank you sir.