Example: Localized Hole: N2+
****************************

"Download ModStPot_N2+.run"

   #! /bin/sh


   # This calculation illustrates:

   # - How to specify the net total charge on a molecule
   # - How to enforce breaking the symmetry that is present in the start-up
   #   situation, in this case to localize a hole in the electron density on one of
   #   the two equivalent atoms.
   # - How to prevent the scf from oscillating back and forth between the two
   #   equivalent situations or from even restoring the unwanted symmetry


   $AMSBIN/ams <<eor
   System
     Atoms
        N 0 0  -2.0
        N 0 0   2.0
     End
     Charge 1
   End

   Task SinglePoint

   Engine ADF
     Title N2+ hole localization
     IrrepOccupations
        sigma 3 // 1 0 1
        pi    2 // 2
     End

     ModifyStartPotential
        N/1    0.5   0.5
        N/2      4   1
     End
     
     Basis
       Type DZP
       Core Small
       CreateOutput Yes
     End

     SCF
       DIIS
         ok 0.5
       End
     End
     Unrestricted Yes
     SpinPolarization 1
     
     Relativity 
       Level None
     End

     Symmetry C(lin)
   EndEngine
   eor

   # The purpose of this run is to compute the N2+ ion, with the hole localized on
   # one of the atoms. In a very small system like N2+ this is a tricky thing to
   # do. The program has a tendency towards the symmetric solution, with the hole
   # delocalized. A few trial runs, just putting a net +1 charge into the system,
   # will reveal that clearly.

   # To achieve the desired situation we apply the key modifystartpotential to
   # break the symmetry of the initial potential. A potential is generated as if
   # the electronic cloud in the second N fragment is spin-polarized in a ratio 4:1
   # (this precise value is not very relevant), which achieves that initially a
   # non-symmetric solution is obtained. The symmetry must be specified, lest the
   # program determine and use the higher symmetry from the nuclear frame. This
   # would prevent any symmetry breaking altogether.

   # Next, in order to prevent that the system relaxes to the symmetric situation,
   # we apply the keeporbitals option of the occupations key. This fixes the
   # occupied orbitals in the sense that in each scf cycle the program will try to
   # keep the electrons in orbitals that resemble the previously occupied orbitals
   # as much as possible.

   # The key modifystartpotential here demonstrated has a more relevant and less
   # unstable application in larger systems. See the User's Guide for references.
