# 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
title N2+ hole localization
N 0 0 -2.0
N 0 0 2.0
symmetry C(lin) ! allow symmetry breaking
! do not use keeporbitals as it may lead to a wrong configuration
sigma 3 // 1 0 1
pi 2 // 2
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
# 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.