#!/bin/sh


# 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 N2 O, 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.

# 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.

# prepare the Nitrogen core hole restricted to be used as fragment


$ADFBIN/adf -n1 <<eor
title   N atom core hole
ATOMS
 N   0.0    0.0     0.0
end
Basis
 Type TZ2P
 Core None
end
xc
 GGA pw86x pw91c
end
numericalquality good
IrrepOccupations
 s    1 2
 p    3
end
eor

mv TAPE21 t21.N_ch


# Now perform the restricted ground state molecule for analysis later. The
# TAPE21 result file is saved.
# Remark: In the core hole calculation the 2 N atoms will have different fragments.
# Therefore also in this calculation the 2 N atoms are treated with 2 different fragments,
# which is done here by using 2 different labels N.1 and N.2.


$ADFBIN/adf <<eor
title   NNO
noprint sfofragpop fragsfo
xc
 GGA pw86x pw91c
end
numericalquality good
ATOMS
 N.1 0.0    0.0    -1.1284
 N.2 0.0    0.0     0.0
 O   0.0    0.0     1.1841
end
Basis
 Type TZ2P
 Core None
 createoutput None
end
eor

mv TAPE21 NNO.t21


# 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.1 0.0    0.0    -1.1284       f=N_ch
 N.2 0.0    0.0     0.0          
 O   0.0    0.0     1.1841       
end
xc
 GGA pw86x pw91c
end
numericalquality good
Basis
 Type TZ2P
 Core None
 createoutput None
end
fragments
 N_ch  t21.N_ch
end
unrestricted
charge 1 1
IrrepOccupations
 sigma  1 1 1 4 // 1 0 1 4 
 pi     4       // 4
end
eor

mv TAPE21 NNO.unr1.t21


# 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 Chong, D.P. Accurate DFT Calculation of Core-
# Electron Binding Energies in Reviews in Modern Quantum Chemistry, A
# Celebration of the Contributions of R.G. Parr, edited by K.D. Sen (World
# Scientific Publishing Co., Singapore), 1106-1139 (2002) for more detailed
# information on Core-Electron Binding Energies. This reference also contain
# information 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
 GGA pw86x pw91c
end
numericalquality good
restart  
   File NNO.unr1.t21
End
ATOMS
 N.1 0.0    0.0    -1.1284       f=NNO
 N.2 0.0    0.0     0.0          f=NNO
 O   0.0    0.0     1.1841       f=NNO
end
fragments
 NNO      NNO.t21
end
unrestricted
charge 1 1
IrrepOccupations
  sigma  1 1 1 4 // 1 0 1 4
  pi     4       // 4
end
eor

mv TAPE21 NNO.unr1.1.t21
rm logfile


# core hole nr. 2

$ADFBIN/adf <<eor
title NNO unrestricted core hole
noprint sfofragpop fragsfo
xc
 GGA pw86x pw91c
end
numericalquality good
ATOMS
 N.1 0.0    0.0    -1.1284       
 N.2 0.0    0.0     0.0          f=N_ch
 O   0.0    0.0     1.1841       
end
Basis
 Type TZ2P
 Core None
 createoutput None
end
fragments
 N_ch  t21.N_ch
end
unrestricted
charge 1 1
IrrepOccupations
 sigma  1 1 1 4 // 1 0 1 4 
 pi     4       // 4 
end
eor

mv TAPE21 NNO.unr2.t21

# analysis for core hole 2 

$ADFBIN/adf <<eor
title   NNO unr. core hole
noprint sfofragpop fragsfo
xc
 GGA pw86x pw91c
end
numericalquality good
restart  
   File NNO.unr2.t21
End
ATOMS
 N.1 0.0    0.0    -1.1284       f=NNO
 N.2 0.0    0.0     0.0          f=NNO
 O   0.0    0.0     1.1841       f=NNO
end
fragments
 NNO      NNO.t21
end
unrestricted
charge 1 1
IrrepOccupations
  sigma  1 1 1 4 // 1 0 1 4 
  pi     4       // 4 
end
eor

mv TAPE21 NNO.unr3.t21

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