#! /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


$ADFBIN/adf <<eor
title N2+  hole localization

atoms
N 0 0  -2.0
N 0 0   2.0
end

Basis
 Type DZP
 Core Small
End

SCF
  DIIS ok=0.5
END

symmetry C(lin)  ! allow symmetry breaking

unrestricted

IrrepOccupations
  ! do not use keeporbitals as it may lead to a wrong configuration
  sigma 3 // 1 0 1
  pi    2 // 2
end

CHARGE  1  1   ! this duplicates info from "OCCUPATIONS" (check)

modifystartpotential  ! to break the symmetry in the start-up potential
N/1    0.5   0.5
N/2      4   1 
end

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.