[QE-users] Negative Core Electron Binding Energy

[Ayestaran Latorre, Carlos]

Dear QE users,

I am trying to calculate binding energy shifts of N 1s core electrons for comparison with XPS spectra for nitrogen-containing system. For that purpose, I am using two PPs from the QE library: N.pbe-n-rrkjus_psl.1.0.0.UPF and the corresponding 1s core-hole one N.star1s-pbe-rrkjus.UPF.

The standard procedure involves calculating the energy of a system with N electrons and comparing with the equivalent system with N-1 electrons, achieved by using the core hole PP, so that a simple definition of binding energy (energy required to remove the electron) might be BE=E(N-1)-E(N). The problem I am having is that I am getting negative values, I suspect due to some simple error I have overlooked.

As an example, I attach the input for a N2 molecule, relaxed without core-hole, and that now includes a core-hole in one of the atoms. The binding energy predicted by this configuration is (rounding numbers):

BE=E(N-1)-E(N)=-51.47Ry-(-40.87Ry)=-10.62 Ry=-144.53  eV

So even if the sign was flipped the absolute value is very far from the ~400eV of the N1s XPS peak. Any help is appreciated.

Kind regards,

Carlos Ayestarán Latorre

PhD student
Department of Mechanical Engineering
Imperial College London

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