NNO: Core-electron binding energies

Sample directory: adf/CEBE_NNO/

ADF is well suited for calculating Core Electron Binding Energies (CEBEs). In this example it is shown how one can differentiate between the 1s CEBEs of the two non-equivalent nitrogen atoms in N₂O, using a delta-SCF technique. It starts with a regular calculation that has the purpose of preparing a reference TAPE21 file for the NNO molecule, which will later be useful in the energy analysis. The result file is saved to t21.NNO.

The same GGA functional is specified throughout the run. The amount of output is reduced by using some print keys.

$ADFBIN/adf -n1  << eor
create N $ADFRESOURCES/TZ2P/N 
xc
gradients pw86x pw91c
end
end input
eor

mv TAPE21 t21.N

$ADFBIN/adf -n1  << eor
create O $ADFRESOURCES/TZ2P/O 
xc
gradients pw86x pw91c
end
end input
eor

mv TAPE21 t21.O

The prepare the nitrogen atom with a core hole (restricted) will be used as a fragment later. This enables selection of where the core hole should be.

$ADFBIN/adf -n1  << eor
title   N atom core hole

ATOMS
N   0.0    0.0     0.0
end

fragments
N      t21.N
end

xc
gradients pw86x pw91c
end

eprint
SFO noeig noovl
end

occupations
s    1 2
p    3
end

end input
eor

mv TAPE21 t21.N_ch

Now perform the restricted ground state molecule for analysis later. The TAPE21 result file is saved.

$ADFBIN/adf << eor
title   NNO

noprint sfofragpop fragsfo

xc
gradients pw86x pw91c
end

ATOMS
N   0.0    0.0    -1.1284
N   0.0    0.0     0.0
O   0.0    0.0     1.1841
end

fragments
N      t21.N
O      t21.O
end

eprint
SFO noeig noovl
end

end input
eor

mv TAPE21 t21.NNO

Next follow two sets of almost identical calculations in which a 1s electron is removed from one or the other N atom (please note that the deepest s level is associated with the 1s of the oxygen atom). The molecular NNO result file is used as fragment. An unrestricted calculation is done and a positive charge is specified. The final result file for the molecule with the core hole is saved. Then another calculation is done to conveniently obtain the energy with respect to the normal molecule. This is repeated for a core hole on the other N atom.

$ADFBIN/adf <<eor
title NNO unrestricted core hole

noprint sfofragpop fragsfo

ATOMS
N   0.0    0.0    -1.1284       f=N_ch
N   0.0    0.0     0.0          f=N
O   0.0    0.0     1.1841       f=O
end

xc
gradients pw86x pw91c
end

fragments
N_ch  t21.N_ch
N     t21.N
O     t21.O
end

eprint
SFO noeig noovl
end

unrestricted
charge 1 1

occupation
sigma  1 1 1 4 // 1 0 1 4 
pi     4       // 4
end

end input
eor

mv TAPE21 t21.NNO.unr1

In the second calculation the result file of one of the unrestricted NNO calculations is used as restart file, which ensures that the hole stays at its place, because the starting density is already correct. The result file t21.NNO for the normal NNO calculation is specified as fragment to serve as an energy reference. The final Bonding Energy printed by ADF indicates what the CEBE is. However, please check Refs.[Chong, 2002 #1239][Chong, 2002 #1238] for more detailed information on Core-Electron Binding Energies. These references also contain infomration on empirical corrections that may have to be made on the final numbers.

$ADFBIN/adf <<eor
title   NNO unr. core hole

noprint sfofragpop fragsfo

xc
gradients pw86x pw91c
end

restart  t21.NNO.unr1

ATOMS
N   0.0    0.0    -1.1284       f=NNO
N   0.0    0.0     0.0          f=NNO
O   0.0    0.0     1.1841       f=NNO
end

eprint
SFO noeig noovl
end

fragments
NNO      t21.NNO
end

unrestricted
charge 1 1

occupation
  sigma  1 1 1 4 // 1 0 1 4
  pi     4       // 4
end

end input
eor

Similarly, one could easily have prepared an oxygen with a core hole and determined the CEBE of the oxygen 1s atom.